diff --git a/public/admin.js b/public/admin.js
index 8fd0127..d7d7037 100644
--- a/public/admin.js
+++ b/public/admin.js
@@ -489,6 +489,7 @@ function copyText(text) {
 
 let lessonModalGroups = null;
 let lessonModalGroupsPromise = null;
+let lessonGroupTouched = false;
 
 function lessonTodayIso() {
   return todayIso();
@@ -498,6 +499,49 @@ function lessonNowTime() {
   return nowHm();
 }
 
+function lessonDowFromIso(iso) {
+  const m = /^(\d{4})-(\d{2})-(\d{2})$/.exec(String(iso || ''));
+  if (!m) return -1;
+  return new Date(Date.UTC(Number(m[1]), Number(m[2]) - 1, Number(m[3]))).getUTCDay();
+}
+
+function lessonMinutes(hm) {
+  const m = /^\s*(\d{1,2}):(\d{2})/.exec(String(hm || ''));
+  if (!m) return null;
+  return Number(m[1]) * 60 + Number(m[2]);
+}
+
+function lessonGroupForDateTime(isoDate, hmTime) {
+  const groups = lessonModalGroups || [];
+  const dow = lessonDowFromIso(isoDate);
+  const minutes = lessonMinutes(hmTime);
+  if (dow < 0 || minutes == null) return null;
+  let best = null;
+  let bestStart = -1;
+  for (const g of groups) {
+    if (g.day_of_week == null || !g.time_start || !g.time_end) continue;
+    if (Number(g.day_of_week) !== dow) continue;
+    const start = lessonMinutes(g.time_start);
+    const end = lessonMinutes(g.time_end);
+    if (start == null || end == null || end < start) continue;
+    if (minutes < start || minutes > end) continue;
+    if (start > bestStart) { bestStart = start; best = g; }
+  }
+  return best;
+}
+
+function applyLessonGroupAuto(modal) {
+  if (lessonGroupTouched) return null;
+  const groupSel = modal.querySelector('#lessonGroup');
+  if (!groupSel) return null;
+  const g = lessonGroupForDateTime(modal.querySelector('#lessonDate').value, modal.querySelector('#lessonTime').value);
+  if (!g) return null;
+  const value = String(g.id);
+  if (!Array.from(groupSel.options).some(o => o.value === value)) return null;
+  groupSel.value = value;
+  return g;
+}
+
 async function loadLessonGroups() {
   if (lessonModalGroups) return lessonModalGroups;
   if (!lessonModalGroupsPromise) {
@@ -576,8 +620,15 @@ function ensureLessonModal() {
     const id = modal.querySelector('#lessonId').value;
     if (id) openLessonVersions(id);
   });
-  groupSel.addEventListener('change', () => prefetchLessonForGroupDate());
-  modal.querySelector('#lessonDate').addEventListener('change', () => prefetchLessonForGroupDate());
+  groupSel.addEventListener('change', () => { lessonGroupTouched = true; prefetchLessonForGroupDate(); });
+  modal.querySelector('#lessonDate').addEventListener('change', () => {
+    applyLessonGroupAuto(modal);
+    prefetchLessonForGroupDate();
+  });
+  modal.querySelector('#lessonTime').addEventListener('change', () => {
+    applyLessonGroupAuto(modal);
+    prefetchLessonForGroupDate();
+  });
   modal.querySelector('#lessonDupGo').addEventListener('click', () => {
     const id = modal.querySelector('#lessonDup').dataset.id;
     if (!id) return;
@@ -718,6 +769,7 @@ function updateLessonAiRow(modal, r) {
 
 function resetLessonModalFields(modal) {
   lessonDupSeq++;
+  lessonGroupTouched = false;
   modal.querySelector('#lessonId').value = '';
   showLessonDuplicate(modal, null);
   modal.querySelector('#lessonDate').value = lessonTodayIso();
@@ -857,6 +909,7 @@ async function openLessonModal(opts = {}) {
   resetLessonModalFields(modal);
   const wanted = opts.groupId ? String(opts.groupId) : previous;
   if (wanted && groups.some(g => String(g.id) === wanted)) groupSel.value = wanted;
+  if (!opts.id && !opts.groupId) applyLessonGroupAuto(modal);
 
   if (opts.text) {
     const textEl = modal.querySelector('#lessonText');