Here's a formula that should work:
whileprintingrecords;
numbervar days := day(cdate(year(currentdate),month(currentdate),1)-1);
numbervar x;
numbervar daycounter := 0;
datetimevar PrevMonth := dateadd("m",-1, currentdate);
for x:= 1 to days do(
if dayofweek(cdate(year(prevmonth),month(prevmonth),x)) in [2 to 6] then
daycounter := daycounter+1
);
Daycounter
-k