Problem is: php DateTime isn't aware of locales.
The following pht call does the trick (but only caused by the side effect of having proper translations for weekdays elsewhere in the code).
Maybe a better solution is to use locale aware date formating functions?
```
--- a/src/view/phui/calendar/PHUICalendarDayView.php
+++ b/src/view/phui/calendar/PHUICalendarDayView.php
@@ -314,7 +314,7 @@ final class PHUICalendarDayView extends AphrontView {
} else if ($box_start_time == $tomorrow_time) {
$title = pht('Tomorrow');
} else {
- $title = $box_start_time->format('l');
+ $title = pht($box_start_time->format('l'));
}
$sidebar_day_boxes[] = array(
```