Calendar - Why 12 months in a year?
calendars began with people recording time by using natural cycles:
days, lunar cycles (months), and solar cycles (years).
year is 365 days 5 hours 48 minutes 46 seconds long or 365.242199
time between full moons is 29.53 days.
Ancient Egyptians are credited with the first calendar of 12 months,
each consisting of 30 days, comprising a year. They added 5 days
at the end of the year to syncronize somewhat with the solar year.
making all their months an even 30 days, they abandoned trying to
sync with lunar cycles and concentrated instead on aligning with
the solar year.
Egyptians recognized that this calendar didn't quite align with
a actual year. They called the 1461 Egyptian years it took to re-align
with the 1460 solar years a Sothis Period.
the Greek rulers of Egypt under Ptolemy added the concept of a leap
year, adding a day every 4 years. The Romans reinforced this concept
when they later ruled Egypt.