Using locale to determine first day of week.

package-lock.json

@@ -3967,6 +3967,11 @@
         "minimist": "0.0.8"
       }
     },
+    "moment": {
+      "version": "2.24.0",
+      "resolved": "https://registry.npmjs.org/moment/-/moment-2.24.0.tgz",
+      "integrity": "sha512-bV7f+6l2QigeBBZSM/6yTNq4P2fNpSWj/0e7jQcy87A8e7o2nAfP/34/2ky5Vw4B9S446EtIhodAzkFCcR4dQg=="
+    },
     "move-concurrently": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/move-concurrently/-/move-concurrently-1.0.1.tgz",

package.json

@@ -62,6 +62,7 @@
     "@fullcalendar/interaction": "^4.1.0",
     "@fullcalendar/react": "^4.1.0",
     "@fullcalendar/timegrid": "^4.1.0",
+    "moment": "~2.24.0",
     "react": "~16.8.1",
     "react-dom": "~16.8.1"
   },

src/Calendar.tsx

@@ -27,6 +27,8 @@ import interactionPlugin from "@fullcalendar/interaction";
 import FullCalendar from "@fullcalendar/react";
 import timeGridPlugin from "@fullcalendar/timegrid";
 
+import { localeData } from "moment";
+
 import { AddEditDaysOffDialog } from "./AddEditDaysOffDialog";
 import { AddEditEventDialog } from "./AddEditEventDialog";
 import { ICalendarEvent } from "./Contracts";
@@ -199,6 +201,7 @@ class ExtensionContent extends React.Component {
                                             { events: this.freeFormEventSource.getEvents },
                                             { events: this.vsoCapacityEventSource.getEvents }
                                         ]}
+                                        firstDay={localeData(navigator.language).firstDayOfWeek()}
                                         header={false}
                                         height={this.getCalendarHeight()}
                                         plugins={[dayGridPlugin, timeGridPlugin, interactionPlugin]}