I have copied the calendar from the Power Apps calendar template to not build it from scratch.
Only the calendar begins with Sunday and it has to begin with Monday. What do i have to change.

/* roll back to the Monday on or before the 1st of the month */
Set(
_firstDayInView;
DateAdd(
_firstDayOfMonth;
-( Weekday( _firstDayOfMonth, StartOfWeek.Monday ) - 1 );
TimeUnit.Days
)
);