Correctly start weeks with Sunday

The days of the week already reflected this, but the headers did not
match.

src/app/calendar/calendar.tsx

@@ -131,7 +131,7 @@ export const CalendarComponent: React.FC<{ calendar: Calendar }> = ({ calendar }
             </header>
             <div className="shadow ring-1 ring-black ring-opacity-5 lg:flex lg:flex-auto lg:flex-col">
                 <div className="grid grid-cols-7 gap-px border-b border-zinc-100 bg-gray-200 text-center text-xs font-semibold leading-6 text-gray-700 lg:flex-none">
-                    {[['M', 'on'], ['T', 'ue'], ['W', 'ed'], ['T', 'hu'], ['F', 'ri'], ['S', 'at'], ['S', 'un']].map((d) => (
+                    {[['S', 'un'], ['M', 'on'], ['T', 'ue'], ['W', 'ed'], ['T', 'hu'], ['F', 'ri'], ['S', 'at']].map((d) => (
                         <div key={d[0] + d[1]} className="bg-white py-2">
                             {d[0]}<span className="sr-only sm:not-sr-only">{d[1]}</span>
                         </div>