Well, there are 365 days in a year(except on leap years when you have 366 days). There are 7 days per week. So 365/7=52 1/7 weeks per year. Since there are 12 months in one year, we have:
(52 1/7) / 12=4.345....etc. weeks in 1 month!!!!. You thought it was simple and straightforward, right?