30 days hath September....

Why do some months have 30 days and others 31? Because if they didn’t, that knuckle thing wouldn’t work.
Originally, the calendar months were mostly 29 days long, with an average of 29.5 days per month. This worked fine, as the moon takes an average of 29 days to orbit the earth. However, this works out to just 354 days per year, well short of the 365 days (365.2422 days actually) that it takes the earth to revolve around the sun.
The Roman emperor Julius Caesar is mostly responsible for our modern calendar. In order to maintain the relationship of the months to the seasons, Caesar inserted several months into the calendar, making the year 445 days long. (Not what you want to see if you’re a teacher.)
In 46 B.C., responding to protests from the RTU (Roman Teachers Union), Caesar took out one month, shortening the year to 365 days, and adding extra days to some of the months, giving them 31.
But there was still the problem of that leftover .2422 days. Caesar solved this by adding a day to February every four years--Leap Year.
Still, the calendar had its problems. The Julian (Caesar's) calendar was off by 10 days in the year 1582, so the dates of October 5-14 were dropped altogether. Also, it was decided that years divisable by 400 would not have Leap Years.
This new calendar (Gregorian) is the one we use today. It no longer follow the phases of the moon, but it does accurately follow the seasons.


http://curious.astro.cornell.edu

http://www.astro.uu.n/