=DAY(A1+15-WEEKDAY(A1-6))<8

Which is another way of saying the if NEXT saturday of A1 date is the last
saturday of the month, the NEXT one (7 days later) will be in the first 7
days (<8) of next month.

Thanks...it works but I don't fully understand how the formula get the
result. Could you tell me more about it?
Why +15 and WEEKDAY(A1-6)?