diff --git a/apps/docs/src/components/date-picker.stories.tsx b/apps/docs/src/components/date-picker.stories.tsx
index 69852b3..f7574c7 100644
--- a/apps/docs/src/components/date-picker.stories.tsx
+++ b/apps/docs/src/components/date-picker.stories.tsx
@@ -37,7 +37,7 @@ function DatePickerPlayground() {
 
 function DatePickerScenarios() {
   return (
-    <div className="grid w-full max-w-4xl gap-4 md:grid-cols-2">
+    <div className="grid w-full max-w-6xl gap-4 md:grid-cols-2 xl:grid-cols-3">
       <Card>
         <CardHeader>
           <CardTitle>Empty state</CardTitle>
@@ -48,6 +48,24 @@ function DatePickerScenarios() {
         </CardContent>
       </Card>
 
+      <Card>
+        <CardHeader>
+          <CardTitle>Sunday-first calendar</CardTitle>
+          <CardDescription>
+            Opt into Sunday-through-Saturday when matching US-style calendar conventions.
+          </CardDescription>
+        </CardHeader>
+        <CardContent>
+          <DatePicker
+            aria-label="Sunday-first launch date"
+            defaultMonth={new Date(2026, 2, 1)}
+            defaultValue={new Date(2026, 2, 18)}
+            locale="en-US"
+            weekStartsOn="sunday"
+          />
+        </CardContent>
+      </Card>
+
       <Card>
         <CardHeader>
           <CardTitle>Guardrailed window</CardTitle>
@@ -76,7 +94,7 @@ const meta = {
     docs: {
       description: {
         component:
-          "A single-date picker for launch windows, review deadlines, and operator scheduling surfaces. This first slice stays intentionally narrow: one date, one popover calendar, no range or timezone API."
+          "A single-date picker for launch windows, review deadlines, and operator scheduling surfaces. It defaults to a Monday-through-Sunday calendar, with an opt-in Sunday-first mode for teams that need that convention. This first slice stays intentionally narrow: one date, one popover calendar, no range or timezone API."
       }
     },
     layout: "centered"
diff --git a/packages/ui/src/components/date-picker.test.tsx b/packages/ui/src/components/date-picker.test.tsx
index 287ba3c..12ef1ad 100644
--- a/packages/ui/src/components/date-picker.test.tsx
+++ b/packages/ui/src/components/date-picker.test.tsx
@@ -86,6 +86,39 @@ describe("DatePicker", () => {
     expect(screen.getByText("April 2028")).toBeInTheDocument();
   });
 
+  it("defaults to a monday-first calendar grid", () => {
+    render(
+      <DatePicker
+        aria-label="Monday-first date"
+        defaultMonth={new Date(2026, 2, 1)}
+        defaultOpen
+        locale="en-US"
+      />
+    );
+
+    const firstDay = within(screen.getByRole("grid")).getAllByRole("gridcell")[0];
+
+    expect(firstDay).toHaveAccessibleName("Feb 23, 2026");
+    expect(screen.getByText("Mon")).toBeInTheDocument();
+  });
+
+  it("supports sunday-first calendar grids when requested", () => {
+    render(
+      <DatePicker
+        aria-label="Sunday-first date"
+        defaultMonth={new Date(2026, 2, 1)}
+        defaultOpen
+        locale="en-US"
+        weekStartsOn="sunday"
+      />
+    );
+
+    const firstDay = within(screen.getByRole("grid")).getAllByRole("gridcell")[0];
+
+    expect(firstDay).toHaveAccessibleName("Mar 1, 2026");
+    expect(screen.getByText("Sun")).toBeInTheDocument();
+  });
+
   it("respects min and max dates", async () => {
     render(
       <DatePicker
diff --git a/packages/ui/src/components/date-picker.tsx b/packages/ui/src/components/date-picker.tsx
index a4d9b2a..40ce3a0 100644
--- a/packages/ui/src/components/date-picker.tsx
+++ b/packages/ui/src/components/date-picker.tsx
@@ -44,6 +44,7 @@ import { createDataAttributes, createSlot } from "../lib/contracts";
 import { ChevronDownIcon, ChevronRightIcon } from "../lib/icons";
 
 type DatePickerValue = Date | undefined;
+export type DatePickerWeekStartsOn = "monday" | "sunday";
 
 function startOfMonth(value: Date) {
   return new Date(value.getFullYear(), value.getMonth(), 1);
@@ -102,9 +103,14 @@ function formatMonthLabel(value: Date, locale?: string) {
   }).format(value);
 }
 
-function buildMonthGrid(month: Date) {
+function getWeekStartIndex(weekStartsOn: DatePickerWeekStartsOn) {
+  return weekStartsOn === "monday" ? 1 : 0;
+}
+
+function buildMonthGrid(month: Date, weekStartsOn: DatePickerWeekStartsOn) {
   const firstDay = startOfMonth(month);
-  const startOffset = firstDay.getDay();
+  const weekStartIndex = getWeekStartIndex(weekStartsOn);
+  const startOffset = (firstDay.getDay() - weekStartIndex + 7) % 7;
   const gridStart = new Date(firstDay);
   gridStart.setDate(firstDay.getDate() - startOffset);
 
@@ -198,6 +204,7 @@ export type DatePickerProps = Omit<
   placeholder?: string;
   todayLabel?: ReactNode;
   value?: Date;
+  weekStartsOn?: DatePickerWeekStartsOn;
 };
 
 export const DatePicker = forwardRef<HTMLInputElement, DatePickerProps>(function DatePicker(
@@ -220,6 +227,7 @@ export const DatePicker = forwardRef<HTMLInputElement, DatePickerProps>(function
     placeholder = "Select date",
     todayLabel = "Today",
     value,
+    weekStartsOn = "monday",
     ...props
   },
   ref
@@ -260,15 +268,18 @@ export const DatePicker = forwardRef<HTMLInputElement, DatePickerProps>(function
     const monthLabel = formatMonthLabel(visibleMonth, locale);
     const weekdays = useMemo(() => {
       const formatter = new Intl.DateTimeFormat(locale, { weekday: "short" });
-      const base = new Date(2025, 0, 5);
+      const base = new Date(2025, 0, 5 + getWeekStartIndex(weekStartsOn));
 
       return Array.from({ length: 7 }, (_, index) => {
         const day = new Date(base);
         day.setDate(base.getDate() + index);
         return formatter.format(day);
       });
-    }, [locale]);
-    const days = useMemo(() => buildMonthGrid(visibleMonth), [visibleMonth]);
+    }, [locale, weekStartsOn]);
+    const days = useMemo(
+      () => buildMonthGrid(visibleMonth, weekStartsOn),
+      [visibleMonth, weekStartsOn]
+    );
     const yearOptions = useMemo(
       () => getYearOptions(visibleMonth, selectedDate),
       [selectedDate, visibleMonth]
@@ -420,161 +431,165 @@ export const DatePicker = forwardRef<HTMLInputElement, DatePickerProps>(function
           </PopoverAnchor>
 
           <PopoverContent className={datePickerContentVariants()} padding="sm" size="xl">
-            <div {...createSlot("header")} className={datePickerHeaderVariants()}>
-              <div className={datePickerNavigationVariants()}>
-                <Button
-                  aria-label="Previous month"
-                  size="icon"
-                  variant="ghost"
-                  onClick={() => {
-                    goToMonth(-1);
-                  }}
-                >
-                  <ChevronRightIcon className="size-3.5 rotate-180" />
-                </Button>
-              </div>
-
-              <div className={datePickerSelectorsVariants()}>
-                <Select
-                  value={String(visibleMonth.getMonth())}
-                  onValueChange={(nextValue) => {
-                    const nextMonth = new Date(visibleMonth.getFullYear(), Number(nextValue), 1);
-                    setVisibleMonth(nextMonth);
-                    onMonthChange?.(nextMonth);
-                  }}
-                >
-                  <SelectTrigger aria-label="Month" className="w-full">
-                    <SelectValue placeholder={monthLabel} />
-                  </SelectTrigger>
-                  <SelectContent>
-                    {Array.from({ length: 12 }, (_, monthIndex) => {
-                      const monthName = new Intl.DateTimeFormat(locale, {
-                        month: "long"
-                      }).format(new Date(visibleMonth.getFullYear(), monthIndex, 1));
-
-                      return (
-                        <SelectItem key={monthIndex} value={String(monthIndex)}>
-                          {monthName}
-                        </SelectItem>
-                      );
-                    })}
-                  </SelectContent>
-                </Select>
-
-                <Select
-                  value={String(visibleMonth.getFullYear())}
-                  onValueChange={(nextValue) => {
-                    const nextMonth = new Date(Number(nextValue), visibleMonth.getMonth(), 1);
-                    setVisibleMonth(nextMonth);
-                    onMonthChange?.(nextMonth);
-                  }}
-                >
-                  <SelectTrigger aria-label="Year" className="w-full">
-                    <SelectValue placeholder={String(visibleMonth.getFullYear())} />
-                  </SelectTrigger>
-                  <SelectContent>
-                    {yearOptions.map((year) => (
-                      <SelectItem key={year} value={String(year)}>
-                        {year}
-                      </SelectItem>
-                    ))}
-                  </SelectContent>
-                </Select>
-              </div>
-
-              <div className={datePickerNavigationVariants()}>
-                <Button
-                  aria-label="Next month"
-                  size="icon"
-                  variant="ghost"
-                  onClick={() => {
-                    goToMonth(1);
-                  }}
-                >
-                  <ChevronRightIcon className="size-3.5" />
-                </Button>
-              </div>
-            </div>
-
-            <p className={datePickerCaptionVariants()}>{monthLabel}</p>
-
-            <div className={datePickerWeekdayVariants()}>
-              {weekdays.map((weekday) => (
-                <span key={weekday}>{weekday}</span>
-              ))}
-            </div>
-
-            <div className={datePickerGridVariants()} role="grid">
-              {days.map((day, index) => {
-                const outside = day.getMonth() !== visibleMonth.getMonth();
-                const selected = sameDay(day, selectedDate);
-                const isToday = sameDay(day, today);
-                const dayDisabled = isDateDisabled(day, minDate, maxDate);
-
-                return (
-                  <button
-                    key={day.toISOString()}
-                    {...createSlot("day")}
-                    {...createDataAttributes({
-                      disabled: dayDisabled,
-                      outside,
-                      selected,
-                      today: isToday
-                    })}
-                    aria-label={formatValue(day, locale)}
-                    aria-pressed={selected}
-                    className={datePickerDayVariants()}
-                    disabled={dayDisabled}
+            <div className="grid gap-3">
+              <div {...createSlot("header")} className={datePickerHeaderVariants()}>
+                <div className={datePickerNavigationVariants()}>
+                  <Button
+                    aria-label="Previous month"
+                    size="icon"
+                    variant="ghost"
                     onClick={() => {
-                      setSelectedDate(normalizeDate(day));
-                      setOpenState(false);
+                      goToMonth(-1);
                     }}
-                    onKeyDown={(event) => handleDayKeyDown(event, index)}
-                    ref={(node) => {
-                      dayRefs.current[index] = node;
-                    }}
-                    role="gridcell"
-                    type="button"
                   >
-                    {day.getDate()}
-                  </button>
-                );
-              })}
-            </div>
+                    <ChevronRightIcon className="size-3.5 rotate-180" />
+                  </Button>
+                </div>
 
-            <div className={datePickerFooterVariants()}>
-              <Button
-                size="sm"
-                variant="subtle"
-                onClick={() => {
-                  setSelectedDate(today);
-                  if (today) {
-                    setVisibleMonth(startOfMonth(today));
-                  }
-                  setOpenState(false);
-                }}
-              >
-                {todayLabel}
-              </Button>
-              <div className="flex items-center gap-2">
+                <div className={datePickerSelectorsVariants()}>
+                  <Select
+                    value={String(visibleMonth.getMonth())}
+                    onValueChange={(nextValue) => {
+                      const nextMonth = new Date(visibleMonth.getFullYear(), Number(nextValue), 1);
+                      setVisibleMonth(nextMonth);
+                      onMonthChange?.(nextMonth);
+                    }}
+                  >
+                    <SelectTrigger aria-label="Month" className="w-full">
+                      <SelectValue placeholder={monthLabel} />
+                    </SelectTrigger>
+                    <SelectContent>
+                      {Array.from({ length: 12 }, (_, monthIndex) => {
+                        const monthName = new Intl.DateTimeFormat(locale, {
+                          month: "long"
+                        }).format(new Date(visibleMonth.getFullYear(), monthIndex, 1));
+
+                        return (
+                          <SelectItem key={monthIndex} value={String(monthIndex)}>
+                            {monthName}
+                          </SelectItem>
+                        );
+                      })}
+                    </SelectContent>
+                  </Select>
+
+                  <Select
+                    value={String(visibleMonth.getFullYear())}
+                    onValueChange={(nextValue) => {
+                      const nextMonth = new Date(Number(nextValue), visibleMonth.getMonth(), 1);
+                      setVisibleMonth(nextMonth);
+                      onMonthChange?.(nextMonth);
+                    }}
+                  >
+                    <SelectTrigger aria-label="Year" className="w-full">
+                      <SelectValue placeholder={String(visibleMonth.getFullYear())} />
+                    </SelectTrigger>
+                    <SelectContent>
+                      {yearOptions.map((year) => (
+                        <SelectItem key={year} value={String(year)}>
+                          {year}
+                        </SelectItem>
+                      ))}
+                    </SelectContent>
+                  </Select>
+                </div>
+
+                <div className={datePickerNavigationVariants()}>
+                  <Button
+                    aria-label="Next month"
+                    size="icon"
+                    variant="ghost"
+                    onClick={() => {
+                      goToMonth(1);
+                    }}
+                  >
+                    <ChevronRightIcon className="size-3.5" />
+                  </Button>
+                </div>
+              </div>
+
+              <div className="grid gap-2">
+                <p className={datePickerCaptionVariants()}>{monthLabel}</p>
+
+                <div className={datePickerWeekdayVariants()}>
+                  {weekdays.map((weekday) => (
+                    <span key={weekday}>{weekday}</span>
+                  ))}
+                </div>
+
+                <div className={datePickerGridVariants()} role="grid">
+                  {days.map((day, index) => {
+                    const outside = day.getMonth() !== visibleMonth.getMonth();
+                    const selected = sameDay(day, selectedDate);
+                    const isToday = sameDay(day, today);
+                    const dayDisabled = isDateDisabled(day, minDate, maxDate);
+
+                    return (
+                      <button
+                        key={day.toISOString()}
+                        {...createSlot("day")}
+                        {...createDataAttributes({
+                          disabled: dayDisabled,
+                          outside,
+                          selected,
+                          today: isToday
+                        })}
+                        aria-label={formatValue(day, locale)}
+                        aria-pressed={selected}
+                        className={datePickerDayVariants()}
+                        disabled={dayDisabled}
+                        onClick={() => {
+                          setSelectedDate(normalizeDate(day));
+                          setOpenState(false);
+                        }}
+                        onKeyDown={(event) => handleDayKeyDown(event, index)}
+                        ref={(node) => {
+                          dayRefs.current[index] = node;
+                        }}
+                        role="gridcell"
+                        type="button"
+                      >
+                        {day.getDate()}
+                      </button>
+                    );
+                  })}
+                </div>
+              </div>
+
+              <div className={datePickerFooterVariants()}>
                 <Button
                   size="sm"
-                  variant="ghost"
-                  onClick={() => {
-                    setSelectedDate(undefined);
-                  }}
-                >
-                  {clearLabel}
-                </Button>
-                <Button
-                  size="sm"
-                  variant="secondary"
+                  variant="subtle"
                   onClick={() => {
+                    setSelectedDate(today);
+                    if (today) {
+                      setVisibleMonth(startOfMonth(today));
+                    }
                     setOpenState(false);
                   }}
                 >
-                  Done
+                  {todayLabel}
                 </Button>
+                <div className="flex items-center gap-2">
+                  <Button
+                    size="sm"
+                    variant="ghost"
+                    onClick={() => {
+                      setSelectedDate(undefined);
+                    }}
+                  >
+                    {clearLabel}
+                  </Button>
+                  <Button
+                    size="sm"
+                    variant="secondary"
+                    onClick={() => {
+                      setOpenState(false);
+                    }}
+                  >
+                    Done
+                  </Button>
+                </div>
               </div>
             </div>
           </PopoverContent>
diff --git a/packages/ui/src/components/date-picker.variants.ts b/packages/ui/src/components/date-picker.variants.ts
index 7f5eda1..01918cf 100644
--- a/packages/ui/src/components/date-picker.variants.ts
+++ b/packages/ui/src/components/date-picker.variants.ts
@@ -8,13 +8,13 @@ export const datePickerFieldVariants = cva("relative");
 export const datePickerTriggerVariants = cva("w-full");
 
 export const datePickerContentVariants = cva([
-  "relative z-50 w-[21rem] overflow-hidden rounded-[var(--ui-panel-radius)] border border-[var(--ui-panel-border)] bg-[var(--ui-panel-bg)] p-0 text-[var(--color-card-foreground)] shadow-[var(--ui-panel-shadow)] outline-none",
+  "relative z-50 w-[22rem] overflow-hidden rounded-[var(--ui-panel-radius)] border border-[var(--ui-panel-border)] bg-[var(--ui-panel-bg)] text-[var(--color-card-foreground)] shadow-[var(--ui-panel-shadow)] outline-none",
   "[border-width:var(--ui-panel-border-width)] backdrop-blur-[var(--ui-panel-backdrop-blur)]",
   "data-[state=open]:motion-enter-rise data-[state=closed]:motion-exit-drop"
 ]);
 
 export const datePickerHeaderVariants = cva(
-  "grid gap-3 sm:grid-cols-[auto_minmax(0,1fr)_auto]"
+  "grid items-center gap-3 sm:grid-cols-[auto_minmax(0,1fr)_auto]"
 );
 
 export const datePickerNavigationVariants = cva("flex items-center gap-2");
@@ -26,7 +26,7 @@ export const datePickerMonthLabelVariants = cva(
 );
 
 export const datePickerCaptionVariants = cva(
-  "px-1 text-sm leading-6 text-[var(--color-muted-foreground)]"
+  "text-sm leading-6 text-[var(--color-muted-foreground)]"
 );
 
 export const datePickerWeekdayVariants = cva(
@@ -50,5 +50,5 @@ export const datePickerDayVariants = cva(
 );
 
 export const datePickerFooterVariants = cva(
-  "flex flex-wrap items-center justify-between gap-3 border-t border-[var(--ui-panel-border)] pt-3"
+  "mt-1 flex flex-wrap items-center justify-between gap-3 border-t border-[var(--ui-panel-border)] pt-3"
 );
diff --git a/packages/ui/src/index.ts b/packages/ui/src/index.ts
index 0f7e18f..7ccb899 100644
--- a/packages/ui/src/index.ts
+++ b/packages/ui/src/index.ts
@@ -148,7 +148,11 @@ export {
   dataTableTableVariants,
   dataTableToolbarVariants
 } from "./components/data-table.variants";
-export { DatePicker, type DatePickerProps } from "./components/date-picker";
+export {
+  DatePicker,
+  type DatePickerProps,
+  type DatePickerWeekStartsOn
+} from "./components/date-picker";
 export {
   datePickerContentVariants,
   datePickerDayVariants,