diff --git a/src/shared/custom.ts b/src/shared/custom.ts
index 05c7260..cb8ecc2 100644
--- a/src/shared/custom.ts
+++ b/src/shared/custom.ts
@@ -1,4 +1,5 @@
 import { z } from "zod";
+import { comesRoundEarly, Repeat } from "./recurrence.ts";
 import { EventType, GachaEvent, Precision, slugify } from "./schema.ts";
 
 /**
@@ -92,6 +93,19 @@ export const CustomEvent = z
     endsAt: z.string().datetime().nullable(),
     endPrecision: Precision,
 
+    /**
+     * How this comes round again, or null when it does not.
+     *
+     * **`.default(null)`, never a bare `.nullable()`.** A record written before
+     * this field existed has no `repeat` key at all, and a bare `.nullable()`
+     * rejects a *missing* key rather than supplying one. `useCustom` reads
+     * through `validRecords`, which drops what fails this schema, and the
+     * survivors are what the next write persists — so the stricter form would
+     * erase every reader's custom events on first launch, silently, with no
+     * server-side copy. The same hazard `game: z.string()` above is guarding.
+     */
+    repeat: Repeat.nullable().default(null),
+
     at: z.string().datetime(),
     updatedAt: z.string().datetime(),
   })
@@ -105,7 +119,23 @@ export const CustomEvent = z
   .refine((e) => e.endsAt === null || e.endsAt > e.startsAt, {
     message: "endsAt must be after startsAt",
     path: ["endsAt"],
-  });
+  })
+  // A window that has not closed by the time it comes round again puts two
+  // live occurrences of one rule in the same list, and "what ends soonest" no
+  // longer has an answer. Checked only when an end is stated: with none, the
+  // window runs to the next opening by definition and cannot overlap.
+  .refine(
+    (e) =>
+      !comesRoundEarly(
+        Date.parse(e.startsAt),
+        e.endsAt === null ? null : Date.parse(e.endsAt),
+        e.repeat,
+      ),
+    {
+      message: "a repeat cannot come round before it ends",
+      path: ["repeat"],
+    },
+  );
 export type CustomEvent = z.infer<typeof CustomEvent>;
 
 export const CustomGames = z.record(z.string(), CustomGame);
diff --git a/test/custom.test.ts b/test/custom.test.ts
index de3ff3a..b84fd35 100644
--- a/test/custom.test.ts
+++ b/test/custom.test.ts
@@ -14,6 +14,7 @@ import {
 } from "../src/shared/custom.ts";
 import { metaFor } from "../src/shared/games.ts";
 import { dailiesId } from "../src/shared/daily.ts";
+import { Repeat } from "../src/shared/recurrence.ts";
 import { eventId, GameId } from "../src/shared/schema.ts";
 import { clockFor } from "../src/shared/time.ts";
 import { readerInstant, validRecords } from "../src/client/state/useCustom.ts";
@@ -397,3 +398,78 @@ describe("retiring a game, a source or a page", () => {
     expect(c.endsMs).not.toBeNull();
   });
 });
+
+describe("a custom event may carry a repeat rule", () => {
+  test("a record stored before this field existed still parses", () => {
+    // THE migration guarantee. readValid drops records that fail this schema,
+    // and the survivors are what the next write persists — so a required or
+    // bare-nullable field here would silently delete every custom event on
+    // every device that has one, with no server-side copy to restore from.
+    const legacy = {
+      id: "myevent:k3f9qa2m01",
+      game: "mygame:limbus-company",
+      title: "Walpurgisnacht",
+      type: "banner",
+      summary: null,
+      startsAt: "2026-08-20T00:00:00.000Z",
+      startPrecision: "day",
+      endsAt: "2026-09-03T00:00:00.000Z",
+      endPrecision: "day",
+      at: AT,
+      updatedAt: AT,
+      // no `repeat` key at all — this is the shape already in localStorage
+    };
+    const parsed = CustomEvent.safeParse(legacy);
+    expect(parsed.success).toBe(true);
+    expect(parsed.data!.repeat).toBe(null);
+  });
+
+  test("accepts a rule whose window closes before it comes round again", () => {
+    const event = ownEvent({
+      startsAt: "2026-09-01T00:00:00.000Z",
+      endsAt: "2026-09-08T00:00:00.000Z",
+      repeat: Repeat.parse({ unit: "weeks", interval: 2, until: null }),
+    });
+    expect(event.repeat?.interval).toBe(2);
+  });
+
+  test("rejects a rule that comes round before it ends", () => {
+    // A 14-day window repeating every 7 days puts two live occurrences of one
+    // rule in the same list, which makes "what ends soonest" ambiguous. Refused
+    // at the schema so an imported file cannot carry one in either.
+    const overlapping = CustomEvent.safeParse({
+      id: "myevent:k3f9qa2m01",
+      game: "mygame:limbus-company",
+      title: "Walpurgisnacht",
+      type: "banner",
+      summary: null,
+      startsAt: "2026-09-01T00:00:00.000Z",
+      startPrecision: "day",
+      endsAt: "2026-09-15T00:00:00.000Z",
+      endPrecision: "day",
+      repeat: { unit: "weeks", interval: 1, until: null },
+      at: AT,
+      updatedAt: AT,
+    });
+    expect(overlapping.success).toBe(false);
+  });
+
+  test("no stated end means there is no overlap to check", () => {
+    // The window runs to the next opening by definition, so it cannot overlap.
+    const parsed = CustomEvent.safeParse({
+      id: "myevent:k3f9qa2m01",
+      game: "mygame:limbus-company",
+      title: "Weekly missions",
+      type: "other",
+      summary: null,
+      startsAt: "2026-09-01T00:00:00.000Z",
+      startPrecision: "day",
+      endsAt: null,
+      endPrecision: "unknown",
+      repeat: { unit: "weeks", interval: 1, until: null },
+      at: AT,
+      updatedAt: AT,
+    });
+    expect(parsed.success).toBe(true);
+  });
+});