Weeks is by the day, such as monday, months is by the date, like the 1st. If the 1st is on a monday, in 4 weeks that Monday will be the 28th, but the month will be the following 1st, which on a 30 day month, would be wednesday. Save yourself the trouble and go by month. Just like how we celebrate ours.

That stuff is confusing! I was honking the same thing yesterday about how I'm going to announce her monthly progressions if I go by weeks.