Hi @Anonymous,
Do you want to change the first day as Monday using the Calendar.WeekdaysShort()?
If you want to achieve this using the Calendar.WeekdaysShort(), I am afraid there is no OOTB in Power Apps. You could directly replace it with [ "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun" ].
Then I will suggest you use the Calendar template within Power Apps, and you should modify something as below:
There is a small change required to the calculation of the first day in view variable
Set(_dateSelected, Today());
Set(_firstDayOfMonth, DateAdd(Today(), 1 - Day(Today()), Days));
Set(_firstDayInView, DateAdd(_firstDayOfMonth, -(Weekday(_firstDayOfMonth) - 2 ), Days));
Set(_lastDayOfMonth, DateAdd(DateAdd(_firstDayOfMonth, 1, Months), -1, Days))
