diff --git a/backend/app/api/_shiftgen.py b/backend/app/api/_shiftgen.py
index 077d9fc..230b824 100644
--- a/backend/app/api/_shiftgen.py
+++ b/backend/app/api/_shiftgen.py
@@ -12,10 +12,22 @@ def _dates(edition: Edition) -> list[dt.date]:
     return [edition.start_date + dt.timedelta(days=i) for i in range(span + 1)]
 
 
-def _window(day: dt.date, part: DayPart) -> tuple[dt.datetime, dt.datetime]:
-    start = dt.datetime.combine(day, part.start_time)
+def _day_boundary(day_parts: list[DayPart]) -> dt.time:
+    """Début du premier créneau type du jour (le plus petit `order`) — même
+    frontière que `validation.engine._day_boundary`. Un créneau type dont
+    l'heure de début tombe avant cette frontière est la suite de la nuit du
+    jour précédent (ex. `Nuit3` 00 h–01 h après `Soirée` 19 h–00 h) : il doit
+    être généré sur le jour *suivant*, pas sur `day` lui-même."""
+    if not day_parts:
+        return dt.time(0, 0)
+    return min(day_parts, key=lambda part: part.order).start_time
+
+
+def _window(day: dt.date, part: DayPart, boundary: dt.time) -> tuple[dt.datetime, dt.datetime]:
+    actual_day = day + dt.timedelta(days=1) if part.start_time < boundary else day
+    start = dt.datetime.combine(actual_day, part.start_time)
     end_day = (
-        day if part.end_time > part.start_time else day + dt.timedelta(days=1)
+        actual_day if part.end_time > part.start_time else actual_day + dt.timedelta(days=1)
     )
     return start, dt.datetime.combine(end_day, part.end_time)
 
@@ -28,6 +40,7 @@ def generate_missing_shifts(
     day_parts = list(
         session.exec(select(DayPart).where(DayPart.edition_id == edition.id))
     )
+    boundary = _day_boundary(day_parts)
     existing = {
         (s.day_part_id, s.starts_at.date())
         for s in session.exec(
@@ -37,9 +50,9 @@ def generate_missing_shifts(
     created = 0
     for day in _dates(edition):
         for part in day_parts:
-            if (part.id, day) in existing:
+            start, end = _window(day, part, boundary)
+            if (part.id, start.date()) in existing:
                 continue
-            start, end = _window(day, part)
             session.add(
                 Shift(
                     edition_id=edition.id,
diff --git a/backend/tests/test_shift_generation.py b/backend/tests/test_shift_generation.py
index a6baf8e..3c123ed 100644
--- a/backend/tests/test_shift_generation.py
+++ b/backend/tests/test_shift_generation.py
@@ -1,5 +1,7 @@
 """Génération des créneaux d'un poste à partir des créneaux types."""
 
+import datetime as dt
+
 
 def _positions_url(edition):
     return f"/editions/{edition.id}/positions"
@@ -58,6 +60,31 @@ def test_adding_a_day_part_then_generating_adds_it_everywhere(client, edition):
     assert resp.json()["created"] == 4  # 1 par jour
 
 
+def test_day_part_before_the_day_boundary_rolls_to_the_next_calendar_day(client, edition):
+    """La frontière du jour est le début du créneau type de plus petit `order`
+    (ici MORNING, 06 h — voir CLAUDE.md § 3 et `validation.engine._day_boundary`).
+    Un créneau type qui commence avant cette heure (ex. `Nuit` 02 h–05 h,
+    suite de `Soirée` 19 h–00 h) est donc la fin de la nuit du jour
+    *précédent* : il doit être généré sur le jour suivant, pas sur `day`."""
+    position = _create_position(client, edition)
+    client.post(
+        f"/editions/{edition.id}/day-parts",
+        json={"label": "Nuit", "start_time": "02:00", "end_time": "05:00", "order": 3},
+    )
+    client.post(f"{_positions_url(edition)}/{position['id']}/generate-shifts")
+
+    shifts = client.get(
+        f"/editions/{edition.id}/shifts", params={"position_id": position["id"]}
+    ).json()
+    nuit_shifts = [s for s in shifts if dt.time.fromisoformat(s["starts_at"][11:19]) == dt.time(2, 0)]
+    assert len(nuit_shifts) == 4
+
+    nuit_dates = sorted(dt.date.fromisoformat(s["starts_at"][:10]) for s in nuit_shifts)
+    # Édition du 9 au 12/07 : la nuit du 9 (Soirée 19 h) se termine le 10 à
+    # 05 h, ..., la nuit du 12 se termine le 13 — jamais le 9 lui-même.
+    assert nuit_dates == [dt.date(2026, 7, d) for d in (10, 11, 12, 13)]
+
+
 def test_generation_does_not_touch_staffing_already_set(client, edition):
     position = _create_position(client, edition)
     shift = client.get(
diff --git a/frontend/src/lib/planning.ts b/frontend/src/lib/planning.ts
index 5499f49..0480e9d 100644
--- a/frontend/src/lib/planning.ts
+++ b/frontend/src/lib/planning.ts
@@ -2,6 +2,7 @@ import type {
   ArtistRead,
   AssignmentRead,
   AvailabilityRead,
+  DayPartRead,
   MatrixResponse,
   PositionCategory,
   ShiftRead,
@@ -10,7 +11,7 @@ import type {
 } from '@/api/schema';
 import { t } from '@/i18n';
 
-import { datePart, hoursBetween, naiveMinutes, timePart } from './time';
+import { addDays, datePart, hoursBetween, minutesOfDay, naiveMinutes, timePart } from './time';
 
 export function categoryLabel(category: PositionCategory): string {
   return t(`position.category.${category.toLowerCase()}`);
@@ -188,35 +189,76 @@ export interface LatticeDay {
   columns: LatticeColumn[];
 }
 
-/** Construit le treillis de colonnes à partir de créneaux (grille alignée). */
-export function buildLattice(starts: Array<{ starts_at: string; ends_at: string }>): {
+/** Frontière du jour (« journée » au sens § 3 du CLAUDE.md) : début du
+ * créneau type de plus petit `order` — même règle que
+ * `validation.engine._day_boundary` côté backend. Sans créneau type, minuit. */
+export function dayBoundaryFromDayParts(
+  dayParts: Array<Pick<DayPartRead, 'start_time' | 'order'>>,
+): string {
+  if (dayParts.length === 0) return '00:00';
+  const first = dayParts.reduce((min, part) => (part.order < min.order ? part : min));
+  return first.start_time.slice(0, 5);
+}
+
+/** Construit le treillis de colonnes à partir de créneaux (grille alignée).
+ * `dayBoundary` (`HH:MM`, défaut minuit) regroupe et trie les colonnes par
+ * « journée de festival » plutôt que par date calendaire brute : un créneau
+ * commençant avant cette heure (ex. 02:00, suite d'une soirée qui a passé
+ * minuit) est rattaché au jour précédent et trié après 23:00, pas avant
+ * 19:00 — voir CLAUDE.md § 3 (« journée » pour le cumul des 8 h). */
+export function buildLattice(
+  starts: Array<{ starts_at: string; ends_at: string }>,
+  dayBoundary = '00:00',
+): {
   days: LatticeDay[];
   columns: LatticeColumn[];
 } {
-  const seen = new Map<string, LatticeColumn>();
+  const boundaryMinutes = minutesOfDay(dayBoundary);
+  const seen = new Map<string, LatticeColumn & { festivalDay: string; sortMinutes: number }>();
   for (const s of starts) {
     const date = datePart(s.starts_at);
     const time = timePart(s.starts_at);
     const endTime = timePart(s.ends_at);
     const key = `${date}|${time}|${endTime}`;
-    if (!seen.has(key)) seen.set(key, { key, date, time, endTime, label: `${time}–${endTime}` });
+    if (!seen.has(key)) {
+      const minutes = minutesOfDay(time);
+      const festivalDay = minutes < boundaryMinutes ? addDays(date, -1) : date;
+      const sortMinutes = (minutes - boundaryMinutes + 24 * 60) % (24 * 60);
+      seen.set(key, { key, date, time, endTime, label: `${time}–${endTime}`, festivalDay, sortMinutes });
+    }
   }
-  const columns = [...seen.values()].sort((a, b) => a.key.localeCompare(b.key));
+  const ordered = [...seen.values()].sort(
+    (a, b) => a.festivalDay.localeCompare(b.festivalDay) || a.sortMinutes - b.sortMinutes,
+  );
+  const columns: LatticeColumn[] = ordered.map(({ key, date, time, endTime, label }) => ({
+    key,
+    date,
+    time,
+    endTime,
+    label,
+  }));
   const days: LatticeDay[] = [];
-  for (const column of columns) {
-    let day = days.find((d) => d.date === column.date);
+  for (const column of ordered) {
+    let day = days.find((d) => d.date === column.festivalDay);
     if (!day) {
-      day = { date: column.date, columns: [] };
+      day = { date: column.festivalDay, columns: [] };
       days.push(day);
     }
-    day.columns.push(column);
+    day.columns.push({
+      key: column.key,
+      date: column.date,
+      time: column.time,
+      endTime: column.endTime,
+      label: column.label,
+    });
   }
   return { days, columns };
 }
 
-export function latticeFromMatrix(matrix: MatrixResponse) {
+export function latticeFromMatrix(matrix: MatrixResponse, dayBoundary?: string) {
   return buildLattice(
     matrix.positions.flatMap((p) => p.shifts.map((s) => ({ starts_at: s.starts_at, ends_at: s.ends_at }))),
+    dayBoundary,
   );
 }
 
diff --git a/frontend/src/lib/time.ts b/frontend/src/lib/time.ts
index 52a6915..e7139db 100644
--- a/frontend/src/lib/time.ts
+++ b/frontend/src/lib/time.ts
@@ -27,6 +27,18 @@ export function timePart(iso: string): string {
   return iso.slice(11, 16);
 }
 
+/** Minutes depuis minuit d'une heure `HH:MM` (ou `HH:MM:SS`). */
+export function minutesOfDay(hhmm: string): number {
+  const [h, m] = hhmm.split(':').map(Number);
+  return h * 60 + m;
+}
+
+/** Décale une date `YYYY-MM-DD` de `delta` jours (peut être négatif). */
+export function addDays(datePartStr: string, delta: number): string {
+  const [y, m, d] = datePartStr.split('-').map(Number);
+  return new Date(Date.UTC(y, m - 1, d + delta)).toISOString().slice(0, 10);
+}
+
 const WEEKDAYS = ['dim.', 'lun.', 'mar.', 'mer.', 'jeu.', 'ven.', 'sam.'];
 const WEEKDAYS_LONG = ['Dimanche', 'Lundi', 'Mardi', 'Mercredi', 'Jeudi', 'Vendredi', 'Samedi'];
 
diff --git a/frontend/src/pages/GrillePage.tsx b/frontend/src/pages/GrillePage.tsx
index 57a23ff..4207be1 100644
--- a/frontend/src/pages/GrillePage.tsx
+++ b/frontend/src/pages/GrillePage.tsx
@@ -1,17 +1,24 @@
 import { useMemo, useState } from 'react';
 
-import { useAssignments, useMatrix, useRunSolver, useVolunteers } from '@/api/hooks';
+import { useAssignments, useDayParts, useMatrix, useRunSolver, useVolunteers } from '@/api/hooks';
 import type { MatrixPosition, MatrixShift, PositionCategory } from '@/api/schema';
 import { Chip, HelpTip } from '@/components/ui';
 import { EmptyNote, ErrorNote, Loading } from '@/components/state';
 import { AssignmentPanel } from '@/pages/grille/AssignmentPanel';
-import { categoryLabel, columnWidthPx, latticeFromMatrix, type LatticeColumn } from '@/lib/planning';
+import {
+  categoryLabel,
+  columnWidthPx,
+  dayBoundaryFromDayParts,
+  latticeFromMatrix,
+  type LatticeColumn,
+} from '@/lib/planning';
 import { datePart, shortDayLabel, timePart } from '@/lib/time';
 import { t } from '@/i18n';
 import { cx } from '@/lib/cx';
 
 export function GrillePage() {
   const matrix = useMatrix();
+  const dayParts = useDayParts();
   const runSolver = useRunSolver();
   const assignments = useAssignments();
   const volunteers = useVolunteers();
@@ -19,9 +26,10 @@ export function GrillePage() {
   const [categoryFilter, setCategoryFilter] = useState<'all' | PositionCategory>('all');
   const [selectedShiftId, setSelectedShiftId] = useState<string | null>(null);
 
+  const dayBoundary = useMemo(() => dayBoundaryFromDayParts(dayParts.data ?? []), [dayParts.data]);
   const lattice = useMemo(
-    () => (matrix.data ? latticeFromMatrix(matrix.data) : { days: [], columns: [] }),
-    [matrix.data],
+    () => (matrix.data ? latticeFromMatrix(matrix.data, dayBoundary) : { days: [], columns: [] }),
+    [matrix.data, dayBoundary],
   );
 
   // Noms des bénévoles affectés, par créneau — pour l'aperçu au survol d'une
diff --git a/frontend/src/pages/VolunteerViewPage.tsx b/frontend/src/pages/VolunteerViewPage.tsx
index 72e068e..7619731 100644
--- a/frontend/src/pages/VolunteerViewPage.tsx
+++ b/frontend/src/pages/VolunteerViewPage.tsx
@@ -1,10 +1,10 @@
 import { useMemo, useState } from 'react';
 
-import { useAssignments, usePositions, useShifts, useVolunteers } from '@/api/hooks';
+import { useAssignments, useDayParts, usePositions, useShifts, useVolunteers } from '@/api/hooks';
 import type { AssignmentRead, ShiftRead, VolunteerRead } from '@/api/schema';
 import { Chip, HelpTip } from '@/components/ui';
 import { EmptyNote, ErrorNote, Loading } from '@/components/state';
-import { buildLattice, hoursByVolunteer, isOverloaded } from '@/lib/planning';
+import { buildLattice, dayBoundaryFromDayParts, hoursByVolunteer, isOverloaded } from '@/lib/planning';
 import { datePart, shortDayLabel, timePart } from '@/lib/time';
 import { t } from '@/i18n';
 import { cx } from '@/lib/cx';
@@ -16,6 +16,7 @@ export function VolunteerViewPage() {
   const assignments = useAssignments();
   const shifts = useShifts();
   const positions = usePositions();
+  const dayParts = useDayParts();
   const [filter, setFilter] = useState<Filter>('all');
 
   const shiftsById = useMemo(
@@ -26,9 +27,10 @@ export function VolunteerViewPage() {
     () => new Map((positions.data ?? []).map((p) => [p.id, p])),
     [positions.data],
   );
+  const dayBoundary = useMemo(() => dayBoundaryFromDayParts(dayParts.data ?? []), [dayParts.data]);
   const lattice = useMemo(
-    () => buildLattice(shifts.data ?? []),
-    [shifts.data],
+    () => buildLattice(shifts.data ?? [], dayBoundary),
+    [shifts.data, dayBoundary],
   );
   const hours = useMemo(
     () => hoursByVolunteer(assignments.data ?? [], shiftsById),