diff --git a/src/utils/date.js b/src/utils/date.js
index 053b025..be9a7a2 100644
--- a/src/utils/date.js
+++ b/src/utils/date.js
@@ -234,13 +234,13 @@ function getLocalizedWeekdayNames(timeZone = DEFAULT_TZ) {
 }
 
 function getLocaleFirstDay() {
-  return new Intl.Locale(navigator.language).weekInfo.firstDay % 7
+  const day = new Intl.Locale(navigator.language).weekInfo?.firstDay ?? 1
+  return day % 7
 }
 
 function getLocaleWeekendDays() {
-  const wk = new Intl.Locale(navigator.language).weekInfo.weekend || [6, 7]
-  const set = new Set(wk.map((d) => d % 7))
-  return Array.from({ length: 7 }, (_, i) => set.has(i))
+  const wk = new Set(new Intl.Locale(navigator.language).weekInfo?.weekend ?? [6, 7])
+  return Array.from({ length: 7 }, (_, i) => wk.has(1 + ((i + 6) % 7)))
 }
 
 function reorderByFirstDay(days, firstDay) {