I have a weekly rate for my clients, but for clients wishing to pay monthly I offer a 3% discount. When I first started doing this it didn't occur to me to take into account the weeks of the year instead of the weeks in a particular month. I finally figured it out and now calculate it for the 52 weeks of the year using a worksheet.
If you would like the specifics check this out:
http://thrivingchildcare.com/2016/03...are-discounts/
Hope that helps.