The original Roman year had 10 named months: Martius (March), Aprilis (April), Maius (May), Junius (June), Quintilis (July), Sextilis (August), September, October, November and December. And probably two unnamed months in the dead of winter when not much happened in agriculture. The year began with Martius.
Numa Pompilius, king of Rome around 700 BC, added the months Januarius (January) and Februarius (February). he also moved the beginning of the year from Martius to Januarius, and changed thenumber of days in several months to be odd, a lucky number. After Februaris three was occasionally an additional month of Intercalaris intercalendar). This is the origin of the leap-year day being in February.
In 46 BC, Julius Ceasar reformed the Roman calender, changing the number of days in many months, and removing Interalaris.
[Addition from Wikipedia:]
The Gregorian calendar was adopted initially by the Catholic countries of Europe, with other countries adopting it over the following centuries. The motivation for the Gregorian reform was that the Julian calendar assumed that the time between vernal equinoxes is 365.25 days, when in fact it was almost 11 minutes shorter. The discrepancy resulted in a drift of about three days every 400 years.
At the time of Gregory’s reform there had already been a drift of 10 days since Roman times, resulting in the spring equinox falling on 11 March instead of the ecclesiastically fixed date of 21 March, and moving steadily earlier in the Julian calendar. Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady movement in the date of the equinox undesirable.