diff --git a/src/components/EventDetail.vue b/src/components/EventDetail.vue
index 9ce48df..4a7254e 100644
--- a/src/components/EventDetail.vue
+++ b/src/components/EventDetail.vue
@@ -1,7 +1,8 @@
 <script setup>
 import { ref, watch } from 'vue'
 import { useCalendar } from '../composables/useCalendar.js'
-import { DOW, DOW_EN } from '../lib/weekday.js'
+import { DOW, DOW_EN, bydayFromDate } from '../lib/weekday.js'
+import { toDate } from '../lib/date.js'
 import OccurrenceGrid from './OccurrenceGrid.vue'
 
 const { selectedEvent, updateEvent, deleteEvent, splitEvent, toggleOccurrence } = useCalendar()
@@ -40,10 +41,16 @@ function applyChanges() {
     dtendTime: form.value.dtend_time,
   }
   if (form.value.is_recurring) {
+    // BYDAY 必须与 dtstartDate 的星期一致, 否则 ICS 输出的 RRULE BYDAY
+    // 与 expandOccurrences 按 dtstartDate 展开的网格不匹配, 导致日历应用
+    // 显示的日期与应用内网格不一致。
+    const byday = form.value.freq === 'WEEKLY'
+      ? bydayFromDate(toDate(ev.dtstartDate))
+      : null
     patch.rrule = {
       freq: form.value.freq,
       interval: Number(form.value.interval) || 1,
-      byday: form.value.freq === 'WEEKLY' ? form.value.byday : null,
+      byday,
       untilDate: form.value.until_date || ev.rrule?.untilDate || '',
     }
   } else {
@@ -139,8 +146,8 @@ function onToggleOcc(date) {
           <input type="date" v-model="form.until_date" />
         </div>
         <div class="form-group" v-if="form.freq === 'WEEKLY'">
-          <label>星期 (BYDAY)</label>
-          <select v-model="form.byday">
+          <label>星期 (BYDAY, 由开始日期自动确定)</label>
+          <select v-model="form.byday" disabled>
             <option v-for="(d, i) in DOW_EN" :key="d" :value="d">
               {{ DOW[i] }}
             </option>