Start calendar weeks on Sunday in the admin display

Render the month grid header Sunday-first with a (weekdayOf + 1) % 7
leading pad, list Sunday first in the weekly-repeat picker, and order
recurrence summaries Sunday-first. The stored Monday=0 .. Sunday=6
weekday numbering is unchanged; a regression test pins it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

admin/src/definitions/DefinitionsTab.test.tsx

@@ -362,6 +362,39 @@ describe("DefinitionsTab", () => {
     expect(screen.getByText("4 definitions · 3 children")).toBeTruthy();
   });
 
+  it("the weekday picker lists Sunday first and Sunday still stores 6", async () => {
+    vi.useFakeTimers({ toFake: ["Date"] });
+    vi.setSystemTime(new Date(2026, 8, 23, 9, 0)); // Wed 23 Sep 2026, local
+    await renderReady();
+
+    fireEvent.click(screen.getByRole("button", { name: "New" }));
+    const s = sheet();
+    fireEvent.change(within(s).getByRole("textbox", { name: "Title" }), {
+      target: { value: "Feed the cat" },
+    });
+    fireEvent.click(within(s).getByRole("button", { name: /Assigned children/ }));
+    fireEvent.click(within(s).getByRole("checkbox", { name: "Declan" }));
+    fireEvent.change(within(s).getByRole("combobox", { name: "Repeats" }), {
+      target: { value: "weekly" },
+    });
+
+    const days = within(s).getByRole("group", { name: "Days" });
+    expect(
+      within(days)
+        .getAllByRole("button")
+        .map((b) => b.getAttribute("aria-label")),
+    ).toEqual(["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]);
+    fireEvent.click(within(days).getByRole("button", { name: "Sunday" }));
+    fireEvent.click(within(s).getByRole("button", { name: "Evening" }));
+
+    await act(async () => {
+      fireEvent.click(within(s).getByRole("button", { name: "Save changes" }));
+    });
+    const body = api.writes()[0].body as { rule: DefinitionRule };
+    expect(body.rule.weekday_set).toEqual([2, 6]);
+    expect(meta(await screen.findByTestId("definition-99"))).toContain("on Sun, Wed");
+  });
+
   it("tapping a row opens the sheet in editing state and saving PATCHes", async () => {
     await renderReady();
     const row = screen.getByTestId("definition-11");

admin/src/definitions/DefinitionsTab.tsx

@@ -27,6 +27,7 @@ import {
 } from "../api/definitions";
 import {
   MONTH_SHORT,
+  WEEKDAY_DISPLAY_ORDER,
   WEEKDAY_SHORT,
   WINDOW_LABELS,
   WINDOW_ORDER,
@@ -680,9 +681,9 @@ function EditSheet({
             </div>
             {draft.repeats === "weekly" || draft.repeats === "custom" ? (
               <div className="defs-weekdays" role="group" aria-label="Days">
-                {WEEKDAY_SHORT.map((label, day) => (
+                {WEEKDAY_DISPLAY_ORDER.map((day) => (
                   <button
-                    key={label}
+                    key={day}
                     type="button"
                     aria-pressed={draft.weekdays.includes(day)}
                     aria-label={WEEKDAY_LONG[day]}
@@ -691,7 +692,7 @@ function EditSheet({
                     }
                     onClick={() => update({ weekdays: toggle(draft.weekdays, day) })}
                   >
-                    {label.charAt(0)}
+                    {WEEKDAY_SHORT[day].charAt(0)}
                   </button>
                 ))}
               </div>

admin/src/definitions/format.ts

@@ -21,6 +21,9 @@ export const WINDOW_LABELS: Record<WindowName, string> = {
 /** Index 0 = Monday .. 6 = Sunday, the core's weekday numbering. */
 export const WEEKDAY_SHORT = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
 
+/** Display order for a week: Sunday first. The stored indices stay Monday=0. */
+export const WEEKDAY_DISPLAY_ORDER = [6, 0, 1, 2, 3, 4, 5];
+
 export const MONTH_SHORT = [
   "Jan", "Feb", "Mar", "Apr", "May", "Jun",
   "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
@@ -51,7 +54,7 @@ export function assigneeSummary(
 
 function weekdayList(days: number[] | null): string {
   return [...(days ?? [])]
-    .sort((a, b) => a - b)
+    .sort((a, b) => WEEKDAY_DISPLAY_ORDER.indexOf(a) - WEEKDAY_DISPLAY_ORDER.indexOf(b))
     .map((day) => WEEKDAY_SHORT[day])
     .join(", ");
 }

admin/src/schedule/ScheduleTab.test.tsx

@@ -131,10 +131,20 @@ describe("ScheduleTab", () => {
     expect(today.className).toContain("schedule-day--today");
     expect(today.dataset.state).toBe("home");
     expect(today.getAttribute("aria-label")).toBe("Sep 23 (today): Home");
-    // September 2026 starts on a Tuesday: one leading pad after the initials.
+    // Sunday-first header; September 2026 starts on a Tuesday: two leading pads.
     const grid = screen.getByRole("group", { name: "Month grid" });
+    expect(Array.from(grid.children, (c) => c.textContent).slice(0, 7)).toEqual([
+      "S",
+      "M",
+      "T",
+      "W",
+      "T",
+      "F",
+      "S",
+    ]);
     expect(grid.children[7].tagName).toBe("SPAN");
-    expect(grid.children[8].getAttribute("data-date")).toBe("2026-09-01");
+    expect(grid.children[8].tagName).toBe("SPAN");
+    expect(grid.children[9].getAttribute("data-date")).toBe("2026-09-01");
 
     const legend = screen.getByRole("list", { name: "Legend" });
     expect(within(legend).getAllByRole("listitem").map((li) => li.textContent)).toEqual([

admin/src/schedule/ScheduleTab.tsx

@@ -46,7 +46,8 @@ export const SCROLL_BOTTOM_PADDING = "92px";
 
 export const DEFAULT_CYCLE_LENGTH_WEEKS = 2;
 const CYCLE_LENGTH_CHOICES = [1, 2, 3, 4];
-const WEEKDAY_INITIALS = ["M", "T", "W", "T", "F", "S", "S"];
+/** Sunday-first header; `weekdayOf` stays Monday=0. */
+const WEEKDAY_INITIALS = ["S", "M", "T", "W", "T", "F", "S"];
 
 export const STATE_LABELS: Record<DayState, string> = {
   home: "Home",
@@ -130,7 +131,7 @@ interface MonthGridProps {
 
 function MonthGrid({ month, today, schedule, effective, overrides, onMonth, onToggle }: MonthGridProps) {
   const dates = monthDates(month);
-  const leading = weekdayOf(month);
+  const leading = (weekdayOf(month) + 1) % 7;
   return (
     <>
       <div className="schedule-month-head">

admin/src/schedule/cycle.test.ts

@@ -58,6 +58,24 @@ describe("calendar helpers", () => {
     expect(weekdayOf("2026-09-06")).toBe(6);
   });
 
+  it("weekdayOf keeps the stored Monday=0 .. Sunday=6 numbering (display order is separate)", () => {
+    // A full known week plus dates across month and year ends.
+    expect(
+      [
+        "2026-09-07",
+        "2026-09-08",
+        "2026-09-09",
+        "2026-09-10",
+        "2026-09-11",
+        "2026-09-12",
+        "2026-09-13",
+      ].map(weekdayOf),
+    ).toEqual([0, 1, 2, 3, 4, 5, 6]);
+    expect(weekdayOf("2026-11-01")).toBe(6); // Sunday
+    expect(weekdayOf("2027-01-01")).toBe(4); // Friday
+    expect(weekdayOf("2024-02-29")).toBe(3); // Thursday
+  });
+
   it("monthStart shifts across year ends", () => {
     expect(monthStart("2026-12-15", 1)).toBe("2027-01-01");
     expect(monthStart("2026-01-31", -1)).toBe("2025-12-01");