Marvin,

I believe that every week should have 7 days, starting on Sunday and ending on Saturday. The only issue then is where week 1 starts. Jerse's stipulation is that week 1 is the first week with most of the days in the relevant month, so that means week 1 starts with the first Sunday that occurs on or after 1st of month -3

Note that my formula should work for any date, A1 doesn't need to be a Wednesday