diff --git a/packages/date-tools/package.json b/packages/date-tools/package.json
new file mode 100644
index 0000000..9d3ac09
--- /dev/null
+++ b/packages/date-tools/package.json
@@ -0,0 +1,26 @@
+{
+  "name": "@batch-cooking/date-tools",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "default": "./dist/index.js"
+    }
+  },
+  "scripts": {
+    "test": "echo \"no tests yet\" && exit 0",
+    "build": "tsc -p tsconfig.json",
+    "postinstall": "tsc -p tsconfig.json"
+  },
+  "devDependencies": {
+    "@types/luxon": "^3.4.2",
+    "typescript": "^5.7.2"
+  },
+  "dependencies": {
+    "luxon": "^3.5.0"
+  }
+}
diff --git a/packages/date-tools/src/date-only.ts b/packages/date-tools/src/date-only.ts
new file mode 100644
index 0000000..05099b9
--- /dev/null
+++ b/packages/date-tools/src/date-only.ts
@@ -0,0 +1,47 @@
+import { DateTime } from "luxon";
+
+// A "date-only" value here always means UTC midnight — Prisma's `@db.Date`
+// columns (`Planning.startDate`/`finishDate`, see apps/api's schema.prisma)
+// carry no time-of-day, so every comparison/computation on them needs to be
+// anchored the same way to stay meaningful. `DateTime` (Luxon) is the
+// in-memory representation everywhere in this package; plain `Date`/ISO
+// `string` only ever appear at the two boundaries that require them —
+// Prisma (`Date`) and URLs/query strings (`string`).
+
+/**
+ * Parses a strict `YYYY-MM-DD` string into a UTC-midnight {@link DateTime}.
+ * Returns `null` for anything that isn't a real calendar date — including a
+ * value that's merely shaped right but impossible (e.g. `2026-02-30`),
+ * unlike native `Date` which would silently roll it over to March 2nd.
+ */
+export function parseDateOnly(iso: string): DateTime | null {
+  const parsed = DateTime.fromISO(iso, { zone: "utc" });
+  return parsed.isValid ? parsed.startOf("day") : null;
+}
+
+/**
+ * Formats a {@link DateTime} back to `YYYY-MM-DD`, the inverse of
+ * {@link parseDateOnly}.
+ *
+ * @throws if `date` is an invalid `DateTime` — every `DateTime` produced by
+ *   this package's own functions is always valid, so this only fires if a
+ *   caller constructs one by hand incorrectly.
+ */
+export function formatDateOnly(date: DateTime): string {
+  const iso = date.toISODate();
+  if (iso === null) {
+    throw new Error("Cannot format an invalid DateTime as a date-only string");
+  }
+  return iso;
+}
+
+/**
+ * Normalizes a `Date` (e.g. a value read back from Prisma) or a
+ * `DateTime` in any zone/with any time-of-day to a UTC-midnight
+ * {@link DateTime} — the common representation every other function in
+ * this package expects and returns.
+ */
+export function toDateOnly(date: Date | DateTime): DateTime {
+  const dateTime = date instanceof DateTime ? date : DateTime.fromJSDate(date, { zone: "utc" });
+  return dateTime.toUTC().startOf("day");
+}
diff --git a/packages/date-tools/src/index.ts b/packages/date-tools/src/index.ts
new file mode 100644
index 0000000..be425aa
--- /dev/null
+++ b/packages/date-tools/src/index.ts
@@ -0,0 +1,12 @@
+// Public entry point of the date-handling utilities shared between apps/api
+// and apps/web — every date computation in the monorepo (parsing/formatting
+// `YYYY-MM-DD` values, week/calendar math) goes through Luxon `DateTime` via
+// this package rather than hand-rolled `Date` arithmetic or a second,
+// differently-behaved date library creeping into one side only.
+
+export * from "./date-only.js";
+export * from "./week.js";
+
+// Re-exported so a consumer never needs its own direct `luxon` dependency
+// just to type a `DateTime` value passed to/from this package's functions.
+export { DateTime } from "luxon";
diff --git a/packages/date-tools/src/week.ts b/packages/date-tools/src/week.ts
new file mode 100644
index 0000000..02690bb
--- /dev/null
+++ b/packages/date-tools/src/week.ts
@@ -0,0 +1,40 @@
+import type { DateTime } from "luxon";
+
+/**
+ * The Monday of the week containing `date` (UTC midnight, same time-of-day
+ * handling as `date-only.ts`). Luxon's `startOf("week")` is Monday-first by
+ * default (ISO 8601 week numbering) regardless of locale, which already
+ * matches the French week this app uses — no locale option needed.
+ */
+export function getWeekStart(date: DateTime): DateTime {
+  return date.startOf("week");
+}
+
+/** Shifts `date` by `n` weeks (negative to go back) — `date` need not already be a week start. */
+export function addWeeks(date: DateTime, n: number): DateTime {
+  return date.plus({ weeks: n });
+}
+
+/**
+ * Builds a fixed 6×7 (weeks × days, Monday-first) calendar grid covering
+ * `month`, the same shape every month-picker UI in this app should use —
+ * always 6 rows regardless of how many weeks the month actually spans, so
+ * the grid never resizes/reflows switching between months. Leading/trailing
+ * days from the adjacent month are included (a caller distinguishes them
+ * with `day.hasSame(month, "month")`), not omitted.
+ */
+export function buildCalendarMonth(month: DateTime): DateTime[][] {
+  const gridStart = getWeekStart(month.startOf("month"));
+
+  const weeks: DateTime[][] = [];
+  let cursor = gridStart;
+  for (let week = 0; week < 6; week++) {
+    const days: DateTime[] = [];
+    for (let day = 0; day < 7; day++) {
+      days.push(cursor);
+      cursor = cursor.plus({ days: 1 });
+    }
+    weeks.push(days);
+  }
+  return weeks;
+}
diff --git a/packages/date-tools/tsconfig.json b/packages/date-tools/tsconfig.json
new file mode 100644
index 0000000..fb47b20
--- /dev/null
+++ b/packages/date-tools/tsconfig.json
@@ -0,0 +1,11 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "declaration": true,
+    "outDir": "dist",
+    "rootDir": "src"
+  },
+  "include": ["src"]
+}
diff --git a/packages/shared/src/index.ts b/packages/shared/src/index.ts
index 95aa7b9..3a7f5e4 100644
--- a/packages/shared/src/index.ts
+++ b/packages/shared/src/index.ts
@@ -7,6 +7,7 @@ export * from "./errors/error-codes.js";
 export * from "./schemas/account.js";
 export * from "./schemas/auth.js";
 export * from "./schemas/household.js";
+export * from "./schemas/planning.js";
 export * from "./schemas/profile.js";
 export * from "./tools/assert-is-never.js";
 export * from "./types/household.js";
diff --git a/packages/shared/src/schemas/planning.ts b/packages/shared/src/schemas/planning.ts
new file mode 100644
index 0000000..31a28ec
--- /dev/null
+++ b/packages/shared/src/schemas/planning.ts
@@ -0,0 +1,17 @@
+import { z } from "zod";
+
+// See schemas/auth.ts for the shared client/server validation rationale.
+
+/**
+ * Payload accepted by `GET /planning`'s `?date=` query param. Only checks
+ * the `YYYY-MM-DD` *shape* — whether it's a real calendar date (e.g.
+ * rejecting `2026-02-30`) is checked service-side via
+ * `@batch-cooking/date-tools`'s `parseDateOnly`, not here: `packages/shared`
+ * has no runtime dependencies of its own, and pulling in a date library just
+ * for this one check isn't worth losing that.
+ */
+export const getPlanningByDateSchema = z.object({
+  date: z.string().regex(/^\d{4}-\d{2}-\d{2}$/, "Date invalide"),
+});
+/** Inferred TS type for {@link getPlanningByDateSchema}'s validated output. */
+export type GetPlanningByDateInput = z.infer<typeof getPlanningByDateSchema>;
diff --git a/packages/shared/src/types/planning.ts b/packages/shared/src/types/planning.ts
index 74a7c99..271b75e 100644
--- a/packages/shared/src/types/planning.ts
+++ b/packages/shared/src/types/planning.ts
@@ -1,3 +1,32 @@
+/**
+ * The 7 values `PlanningItemView.weekDay` is expected to take — lowercase,
+ * unaccented French day names. Not enforced by the database (`week_day` is
+ * a plain `String` column, see schema.prisma) or by any write endpoint yet
+ * (there isn't one), but this is the contract the planning grid
+ * (`apps/web`'s `PlanningPage`) reads against, and the one a future
+ * "add a recipe to a slot" endpoint should write.
+ */
+export const WEEK_DAYS = [
+  "lundi",
+  "mardi",
+  "mercredi",
+  "jeudi",
+  "vendredi",
+  "samedi",
+  "dimanche",
+] as const;
+/** Inferred TS type for one {@link WEEK_DAYS} member. */
+export type WeekDay = (typeof WEEK_DAYS)[number];
+
+/**
+ * The 5 values `PlanningItemView.meal` is expected to take, in day order —
+ * same "documented but not enforced yet" status as {@link WEEK_DAYS}, same
+ * reason.
+ */
+export const MEALS = ["petit-dejeuner", "collation", "dejeuner", "gouter", "diner"] as const;
+/** Inferred TS type for one {@link MEALS} member. */
+export type Meal = (typeof MEALS)[number];
+
 /**
  * A single meal slot within a household's planning, with its recipe
  * resolved to just enough info for display (id + name) — a caller needing
@@ -5,9 +34,9 @@
  */
 export interface PlanningItemView {
   id: number;
-  /** Day of the week this item falls on (free-form for now — no enum exists yet, see schema.prisma). */
+  /** Day of the week this item falls on — see {@link WeekDay} (free-form for now — no enum exists yet, see schema.prisma). */
   weekDay: string;
-  /** Which meal of the day this item is for (free-form for now, same reason). */
+  /** Which meal of the day this item is for — see {@link Meal} (free-form for now, same reason). */
   meal: string;
   recipe: {
     id: number;
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index b2efbb3..a842ad5 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -17,6 +17,9 @@ importers:
 
   apps/api:
     dependencies:
+      '@batch-cooking/date-tools':
+        specifier: workspace:*
+        version: link:../../packages/date-tools
       '@batch-cooking/error-tools':
         specifier: workspace:*
         version: link:../../packages/error-tools
@@ -87,6 +90,9 @@ importers:
 
   apps/web:
     dependencies:
+      '@batch-cooking/date-tools':
+        specifier: workspace:*
+        version: link:../../packages/date-tools
       '@batch-cooking/shared':
         specifier: workspace:*
         version: link:../../packages/shared
@@ -137,6 +143,19 @@ importers:
         specifier: ^5.4.11
         version: 5.4.21(@types/node@22.20.1)(sass@1.102.0)
 
+  packages/date-tools:
+    dependencies:
+      luxon:
+        specifier: ^3.5.0
+        version: 3.7.2
+    devDependencies:
+      '@types/luxon':
+        specifier: ^3.4.2
+        version: 3.7.4
+      typescript:
+        specifier: ^5.7.2
+        version: 5.9.3
+
   packages/error-tools:
     dependencies:
       '@batch-cooking/shared':
@@ -1055,6 +1074,9 @@ packages:
   '@types/jsonwebtoken@9.0.10':
     resolution: {integrity: sha512-asx5hIG9Qmf/1oStypjanR7iKTv0gXQ1Ov/jfrX6kS/EO0OFni8orbmGCn0672NHR3kXHwpAwR+B368ZGN/2rA==, tarball: https://registry.npmjs.org/@types/jsonwebtoken/-/jsonwebtoken-9.0.10.tgz}
 
+  '@types/luxon@3.7.4':
+    resolution: {integrity: sha512-V536ZAd6ZJztrrBlLcDFaaZrXNAL2E5uGmssWf/dpSiLkmkLScXUYhUBnWPmtW+cIqnNHzf6//TCMpIc9SCRRQ==, tarball: https://registry.npmjs.org/@types/luxon/-/luxon-3.7.4.tgz}
+
   '@types/methods@1.1.4':
     resolution: {integrity: sha512-ymXWVrDiCxTBE3+RIrrP533E70eA+9qu7zdWoHuOmGujkYtzf4HQF96b8nwHLqhuf4ykX61IGRIB38CC6/sImQ==, tarball: https://registry.npmjs.org/@types/methods/-/methods-1.1.4.tgz}
 
@@ -3617,6 +3639,8 @@ snapshots:
       '@types/ms': 2.1.0
       '@types/node': 22.20.1
 
+  '@types/luxon@3.7.4': {}
+
   '@types/methods@1.1.4': {}
 
   '@types/mime@1.3.5': {}