diff --git a/src/components/CalendarDay.vue b/src/components/CalendarDay.vue
index 7032d6a..b299ee7 100644
--- a/src/components/CalendarDay.vue
+++ b/src/components/CalendarDay.vue
@@ -1,15 +1,61 @@
 <script setup>
-import { computed } from 'vue'
-import { formatDateCompact, fromLocalString } from '@/utils/date'
+import { computed, ref, onMounted, onBeforeUnmount } from 'vue'
+import { fromLocalString } from '@/utils/date'
 
 const props = defineProps({
   day: Object,
   dragging: { type: Boolean, default: false },
 })
 
+// Reactive viewport width detection
+const isNarrowView = ref(false)
+const isVeryNarrowView = ref(false)
+const isSmallView = ref(false)
+
+function checkViewportWidth() {
+  const width = window.innerWidth
+  isSmallView.value = width < 800
+  isNarrowView.value = width < 600
+  isVeryNarrowView.value = width < 400
+}
+
+onMounted(() => {
+  checkViewportWidth()
+  window.addEventListener('resize', checkViewportWidth)
+})
+
+onBeforeUnmount(() => {
+  window.removeEventListener('resize', checkViewportWidth)
+})
+
 const formattedDate = computed(() => {
   const date = fromLocalString(props.day.date)
-  return formatDateCompact(date)
+  
+  let options = { day: 'numeric', month: 'short' }
+  
+  if (isVeryNarrowView.value) {
+    // Very narrow: show only day number
+    options = { day: 'numeric' }
+  } else if (isNarrowView.value) {
+    // Narrow: show day and month, no weekday
+    options = { day: 'numeric', month: 'short' }
+  } else {
+    // Wide: show weekday, day, and month
+    options = { weekday: 'short', day: 'numeric', month: 'short' }
+  }
+  
+  let formatted = date.toLocaleDateString(undefined, options)
+  
+  // Below 700px, replace first space with newline to force weekday on separate line
+  if (isSmallView.value && !isNarrowView.value && !isVeryNarrowView.value) {
+    formatted = formatted.replace(/\s/, '\n')
+  }
+  
+  // Replace the last space (between month and day) with nbsp to prevent breaking there
+  // but keep the space after weekday (if present) as regular space to allow wrapping
+  formatted = formatted.replace(/\s+(?=\S+$)/, '\u00A0')
+  
+  return formatted
 })
 </script>
 
@@ -136,6 +182,7 @@ const formattedDate = computed(() => {
   color: var(--ink);
   line-height: 1;
   pointer-events: none;
+  white-space: pre-wrap;
 }
 
 .cell.weekend .compact-date {
diff --git a/src/utils/date.js b/src/utils/date.js
index 73beddb..00dc76b 100644
--- a/src/utils/date.js
+++ b/src/utils/date.js
@@ -192,17 +192,6 @@ function formatTodayString(date, weekday = "long", month = "long") {
   return formatted.charAt(0).toUpperCase() + formatted.slice(1)
 }
 
-/**
- * Format date as compact string for day cell corner (e.g., "Mon 15 Jan")
- */
-function formatDateCompact(date) {
-  return date.toLocaleDateString(undefined, { 
-    weekday: 'short', 
-    day: 'numeric', 
-    month: 'short'
-  })
-}
-
 export {
   // constants
   monthAbbr,
@@ -229,7 +218,6 @@ export {
   formatDateRange,
   formatDateShort,
   formatDateLong,
-  formatDateCompact,
   formatTodayString,
   lunarPhaseSymbol,
   // iso helpers re-export