diff --git a/web/src/components/ActivityCalendar.tsx b/web/src/components/ActivityCalendar.tsx
index 9aaf3ad0..d59445ce 100644
--- a/web/src/components/ActivityCalendar.tsx
+++ b/web/src/components/ActivityCalendar.tsx
@@ -39,7 +39,7 @@ const ActivityCalendar = (props: Props) => {
   const year = dayjs(monthStr).toDate().getFullYear();
   const month = dayjs(monthStr).toDate().getMonth() + 1;
   const dayInMonth = new Date(year, month, 0).getDate();
-  const firstDay = (new Date(year, month - 1, 1).getDay() - weekStartDayOffset) % 7;
+  const firstDay = ((new Date(year, month - 1, 1).getDay() - weekStartDayOffset) % 7 + 7) % 7;
   const lastDay = new Date(year, month - 1, dayInMonth).getDay() - weekStartDayOffset;
   const WEEK_DAYS = [t("days.sun"), t("days.mon"), t("days.tue"), t("days.wed"), t("days.thu"), t("days.fri"), t("days.sat")];
   const weekDays = WEEK_DAYS.slice(weekStartDayOffset).concat(WEEK_DAYS.slice(0, weekStartDayOffset));