Select
5 rows and 7 columns => Paste formula below with Ctrl
+ Enter
=DATE(YEAR(B2),MONTH(B2),1)-(WEEKDAY(DATE(YEAR(B2),MONTH(B2),1))-1)+{0;1;2;3;4;5}*7+{1,2,3,4,5,6,7}-1
Result:
and then formatting the last month.
=> Select whole calendar => Conditional Formatting =>
