if the cell is a correct date and just formatted to show the month
then use a formula for conditional formatting
select the range A32:M32
then use a formula
=WEEKDAY(A32,2)>5
that will highlight weekends
Not sure what you mean by invalid dates
They will be entered as text and so not a number
then enter a 2nd rule and use a formula
=NOT(ISNUMBER(A32))
that will test to see if the cell contains a number, which a valid date will be