I think this AI-assisted answer is the clearest:

=EOMONTH(A1,0)-MOD(WEEKDAY(EOMONTH(A1,0),2)-5,7)

A1 is a date value with any date in the subject month

-5 is for Friday. For other days:
-4 is for Thursday
etc for the sequence -1 (Monday) through -7 (Sunday)