When and why was it decided 12 months for a calendar when 13 would have been more equally divided?

Question

It would have been thirteen - 28 day months (which I think would have coincided with the moon better), with 1 day needed to make 365.

Answer 1

The Egyptians tried to balance the time between the full moons and fitting it into a solar calendar. 29.53 days is closer to 30 so 30 it was and the they added 5 days at the end of each year to make up for the difference. The Greek while ruling Egypt then added the leap year.

Source:

http://www.calendar-origins.com/egyptian...

As for the number of days in each month, it has to do with the Romans their deities and their rulers vanity.

The months dedicated to a Roman god or emperor have 31 days but there are others too, most likely they were important in a religious way.

Mars (god) 31, it was the first month in the Roman year.

April 30.

May (goddess) 31.

June 30.

July (Julius Cesar) 31. (He took from February.)

August (Augustus) 31. (He took from February.)

September 30. (Septem =7)

October 31, since the rule of Augustus. (The name October comes from the Latin word for 8)

November 30. (Novem = 9)

December 31. (decem = 10)

January (god) 31 .

February 28 (29). February used to have 30 days but Julius and Augustus took one day each from it to add to their own months July and August since none of them wanted to have less days than any other of the important months.

October was harvest time and could have it's 31 days dedicated to gods that has to do with harvest, prosperity and such to do.

December was the time for the festival called Saturnalia in honor of the god Saturn.  

Answer 2

A month is the time period of one moon cycle. 12 moon cycles = one rotation of the Earth around the sun.