diff --git a/client/src/mobile/screens/trip/MTripShell.tsx b/client/src/mobile/screens/trip/MTripShell.tsx
index 8daff7378..61cb96c17 100644
--- a/client/src/mobile/screens/trip/MTripShell.tsx
+++ b/client/src/mobile/screens/trip/MTripShell.tsx
@@ -1,5 +1,6 @@
 import { useEffect, useRef, useState, type ComponentType, type ReactNode } from 'react'
 import { findFocusDayId, findTodayDayId } from '../../../components/Planner/today'
+import { takeResumeDay } from '../../../utils/resumeRoute'
 import {
   CalendarCheck, CalendarDays, ChevronDown, ChevronLeft, Download, FileDown, FolderSync, List, Map as MapIcon, MoreHorizontal, Plane, Plus, Route, Rows3, SlidersHorizontal, Trash2, Upload,
 } from 'lucide-react'
@@ -256,8 +257,11 @@ export default function MTripShell({
     seededDayRef.current = true
     // Off the same helper file as the desktop day plan (#1567), so the two
     // cannot drift on what "today" means.
-    planner.tripActions.setSelectedDay(findFocusDayId(days) ?? days[0].id)
-  }, [isLoading, planner.selectedDayId, days, planner.tripActions])
+    // A relaunch after the system closed the app goes back to the day it was on (#666).
+    const resumed = takeResumeDay(tripId)
+    const resumedDay = resumed != null && days.some(d => d.id === resumed) ? resumed : null
+    planner.tripActions.setSelectedDay(resumedDay ?? findFocusDayId(days) ?? days[0].id)
+  }, [isLoading, planner.selectedDayId, days, planner.tripActions, tripId])
 
   // Swiping the day panel (#2051) can move the day well past the chips on
   // screen — the rail overflows from roughly six days on — so the active chip
diff --git a/client/src/utils/resumeRoute.test.tsx b/client/src/utils/resumeRoute.test.tsx
index 040ebc7e6..e92bc9ecd 100644
--- a/client/src/utils/resumeRoute.test.tsx
+++ b/client/src/utils/resumeRoute.test.tsx
@@ -3,7 +3,7 @@ import { beforeEach, describe, expect, it } from 'vitest'
 import { renderHook } from '@testing-library/react'
 import { MemoryRouter } from 'react-router'
 import type { ReactNode } from 'react'
-import { forgetResumeRoute, rememberRoute, RESUME_MAX_AGE_MS, takeResumeRoute, useRememberRoute } from './resumeRoute'
+import { forgetResumeRoute, rememberRoute, RESUME_MAX_AGE_MS, takeResumeDay, takeResumeRoute, useRememberRoute } from './resumeRoute'
 
 const NOW = 1_800_000_000_000
 
@@ -58,4 +58,14 @@ describe('resume route (#1024)', () => {
     window.dispatchEvent(new Event('pagehide'))
     expect(JSON.parse(localStorage.getItem('trek:resume-route')!).path).toBe('/journey/9')
   })
+  it('FE-UTIL-RESUME-666: the plan day comes back once, for its own trip only', () => {
+    rememberRoute('/trips/7', NOW, 55)
+    sessionStorage.clear()
+    takeResumeRoute(NOW, true)
+    expect(takeResumeDay(8)).toBeNull()
+    expect(takeResumeDay(7)).toBe(55)
+    expect(takeResumeDay(7)).toBeNull()
+    rememberRoute('/journey/1', NOW, 55)
+    expect(JSON.parse(localStorage.getItem('trek:resume-route')!).day).toBeUndefined()
+  })
 })
diff --git a/client/src/utils/resumeRoute.ts b/client/src/utils/resumeRoute.ts
index b4721780f..0194d081c 100644
--- a/client/src/utils/resumeRoute.ts
+++ b/client/src/utils/resumeRoute.ts
@@ -1,5 +1,6 @@
 import { useEffect } from 'react'
 import { useLocation } from 'react-router'
+import { useTripStore } from '../store/tripStore'
 
 /**
  * Where the installed app was when it went to the background (#1024).
@@ -14,10 +15,27 @@ import { useLocation } from 'react-router'
  */
 const KEY = 'trek:resume-route'
 const CHECKED_KEY = 'trek:resume-checked'
+const DAY_KEY = 'trek:resume-day-'
+
+/**
+ * The day the plan was on when the app went away (#666), handed over once to
+ * whoever seeds the trip's first day selection. Null when there is none.
+ */
+export function takeResumeDay(tripId: number): number | null {
+  try {
+    const raw = sessionStorage.getItem(`${DAY_KEY}${tripId}`)
+    if (raw == null) return null
+    sessionStorage.removeItem(`${DAY_KEY}${tripId}`)
+    const day = Number(raw)
+    return Number.isInteger(day) ? day : null
+  } catch {
+    return null
+  }
+}
 /** Older than this, a relaunch is a fresh start again. */
 export const RESUME_MAX_AGE_MS = 6 * 60 * 60 * 1000
 
-interface ResumeRecord { path: string; tab?: string; at: number }
+interface ResumeRecord { path: string; tab?: string; day?: number; at: number }
 
 /** True in a display mode that has no tab strip, i.e. launched from the home screen. */
 export function isInstalledApp(): boolean {
@@ -45,7 +63,7 @@ function withoutTabParam(path: string): string {
 }
 
 /** Called on every protected navigation and when the app is hidden. */
-export function rememberRoute(path: string, now = Date.now()): void {
+export function rememberRoute(path: string, now = Date.now(), day: number | null = null): void {
   try {
     if (!resumable(path)) {
       localStorage.removeItem(KEY)
@@ -57,7 +75,7 @@ export function rememberRoute(path: string, now = Date.now()): void {
     const asked = new URLSearchParams(path.split('?')[1] ?? '').get('tab') ?? undefined
     const tab = tripId ? sessionStorage.getItem(`trip-tab-${tripId}`) ?? asked : undefined
     const kept = tripId ? withoutTabParam(path) : path
-    const record: ResumeRecord = { path: kept, at: now, ...(tab ? { tab } : {}) }
+    const record: ResumeRecord = { path: kept, at: now, ...(tab ? { tab } : {}), ...(tripId && day != null ? { day } : {}) }
     localStorage.setItem(KEY, JSON.stringify(record))
   } catch {
     // Storage refused: the relaunch simply starts where it always did.
@@ -83,6 +101,7 @@ export function takeResumeRoute(now = Date.now(), installed = isInstalledApp()):
     if (now - record.at > RESUME_MAX_AGE_MS || !resumable(record.path)) return null
     const tripId = tripIdOf(record.path)
     if (tripId && record.tab) sessionStorage.setItem(`trip-tab-${tripId}`, record.tab)
+    if (tripId && typeof record.day === 'number') sessionStorage.setItem(`${DAY_KEY}${tripId}`, String(record.day))
     return record.path
   } catch {
     return null
@@ -102,9 +121,10 @@ export function useRememberRoute(): void {
   const location = useLocation()
   const path = location.pathname + location.search
   useEffect(() => {
-    rememberRoute(path)
-    const onVisibility = () => { if (document.visibilityState === 'hidden') rememberRoute(path) }
-    const onPageHide = () => rememberRoute(path)
+    const save = () => rememberRoute(path, Date.now(), useTripStore.getState().selectedDayId ?? null)
+    save()
+    const onVisibility = () => { if (document.visibilityState === 'hidden') save() }
+    const onPageHide = () => save()
     document.addEventListener('visibilitychange', onVisibility)
     window.addEventListener('pagehide', onPageHide)
     return () => {