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Before Julius Caesar reformed the Roman calendar and gave the year 365 days (with an extra day every fourth year), the Romans had a 12-month calendar but inserted an extra month every so often to keep the years in line with the solar year, more or less (at the time of Caesar's reform they had somehow gotten 90 days off).

March was originally the first month and some of the month names show this--September was the 7th month (Latin septem means seven), October was the eighth month (octo = eight), etc. Later they began to treat January 1st as the beginning of the year.

July and August were called Quintilis and Sextilis before being renamed in honor of Julius Caesar and Augustus Caesar. Some later emperors had months named for them but the names didn't last.