I am not a VB Scripter, but I hope the logic in this VBA code is helpful.
Dim lngLastDay as long
Dim blnShowOnEvenDays as boolean
' Determine the last day of the previous month
lngLastDay = DateSerial(Year(Now), Month(Now), 1) - 1
' If the last day of the previous month was 29 or 31 (odd)...