How can i calculate the first sunday of each month
 

Would it be this if I want it to do it for all rows and the A column has all the dates to be validated?

=DATE(YEAR($A:$A), MONTH($A:$A),1)+6-WEEKDAY(DATE(YEAR($A:$A), MONTH($A:$A), 1),3)

How can i calculate the first sunday of each month
 

This worked much better:

=DAY($A:$A)<8