message: Use index as key for weekdays
  The luxon utils return single-character weekdays, which gives us duplicate keys
  T and S.
diff: @@ -59,9 +59,9 @@ export const CalendarHeader = ({
  <div className={classes.daysHeader}>
  {
  - utils.getWeekdays().map(day => (
  + utils.getWeekdays().map((day, index) => (
  <Typography
  - key={day}
  + key={index}
  variant="caption"
  className={classes.dayLabel}
  >
  