How to Calculate Months in Roman Inclusive Counting

There are three fixed points in each month. The nonas is sometimes the seventh day of the month and sometimes the fifth day, so we call it nonas. And the ides, or edos, is the fifteenth day of themonth and sometimes the thirteenthday of the month. But why do you even have fixed days that aren't fixed its? Because the months themselves are different lengths,. So longer months, the edas is the fifteen day of longer months, 31 days because e days means the same got 15 days either side.