As a Roman Polytheist, the festival calendar that I follow is based on Roman civic and agricultural holidays. In doing so, I realized that the Gregorian calendar, which is in use today, carried elements of the old Roman one. For example, the names of the months are the same. Furthermore, the number of days (30 or 31) in a month was set by Julius Caesar and the Roman Senate. The ordering of modern time seems to be based on Roman ideas with February as a traditional short month.
“December” indicates that the original Roman calendar had only ten months. The year ended in December with a 62 day period attached to the end. At that time, January and February did not exist as separate months. The year began with March (named for Mars) and continued through April (meaning to Open), May (Maia), June (Juno), Quintilis (5th), Sextilis (6th), September (7th), October (8th), November (9th) and December (10th).
The second king of Rome, Numa Pompilius reformed the 10 month calendar set by Romulus. He added January (named for Janus) and February (meaning to Purify) at the beginning of the year. His calendar, a lunar one, had an intercalary month which fell between February and March. This month (Intercalaris or Mercedinus) was added by the Pontifex Maximus (high priest) as needed to align the calendar with the seasons.
Numa’s calendar featured “hollow months” of 29 days and “full months” of 30 days. When the intercalary month was used, February was reduced to 23/24 days, while Intercalaris had 27/28 days. In contrast, the calendar of Romulus had six months of 30 days, four months of 31 days and winter of 61 days.
Because of Roman politics, the calendar of Numa became out of sync with the Roman climate. Different administrations did not want festivals to fall on certain dates, while others wanted to extend the year for a particular Consul. By the time, Julius Caesar became the Pontifex Maximus, the calendar was a mess.
Upon the urging of Sosigenes, a Greek astronomer of Alexandria, Caesar decided to create a solar calendar. He would have three years of 365 days and a leap year of 366. Caesar set the months to be 30 or 31 days (alternating). The Senate changed it to have February have only 28 days, July (Julius Caesar) 31 days, and August (Augustus) 31 days. The Julian calendar was in general use until the reforms of Pope Gregory XIII. (The Orthodox Church still uses the Julian calendar.)