system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the moon orbits 13 times a year. So why not have 13 months ?

go, Smarch!

Thanks all.
I was told once it was because there were 12 apostles, but no, it Science !
(thanks again)
gpt
Because it's not exact.

The lunar month is slightly longer than 29.5 days. So, 12 lunar months make a 354-day lunar year. Our 365-day year isn't arbitrary, it is based on the time it takes the Earth to go around the Sun, which affects things like agricultural seasons.

There are some solutions to this difference between the lunar and solar years. The Romans said "screw the Moon", and made the months 30 or 31 days long, so that a 12-month period would equal a solar year; this at the cost of the months having nothing to do with the phases of the Moon any more. Theirs was the basis for our current civil calendar (we just found out they had a bit too many leap years).

The Muslims said "screw the Sun"; their calendar has 12 lunar months and, as such, is shorter than our civil calendar. You can notice this by seeing that their religious celebrations (the holy month of Ramadan, the Hajj pilgrimage to Mecca) happen about 11 days earlier every year. This implies the Muslim calendar has nothing to do with seasons, which wasn't much of a problem for the original desert nomadic Muslims.

We Jews didn't say "screw" anything. Jewish months are lunar-based, alternating between 29 and 30 days. However, in our tradition it was important to keep accurate track of the seasons, so we sometimes insert a thirteenth month in the year to make up for those 11 days the  lunar year is shorter than a solar year. Initially, the decision of how long a month would be, and whether or not to include an extra month in a given year, was made by observation, but afterwards it was standardised in mathematical rules. Given that sometimes we have to add or subtract a day to prevent a year from beginning in some days of the week, the cost of the system is that it is very complicated for someone to know exactly what the calendar for a given year will be!