According to legend the Roman calendar was created by Romulus, the founder of Rome, around 750 BC. It was essentially a lunar based calendar and had ten months. The ten months took a total of 304 days. Each year started in March and ended in December and was then followed by a period of festival between the years. The next year would start on a full moon so that the lunar cycle would be back in its proper phase. This did not adjust the solar cycle however and this led to constant calendar revisions over the years.
Any wars were supposed to be suspended during the period of festival between the years, and could then be continued in the new year. It may have been for this reason that the first month of the old calendar was called March, for Mars, the Roman God of war.
The name April has a less certain origin. It has been suggested that it is from the old Roman term for after, or second. Another source suggests it was a shortened form of the Greek and Etruscan month name for Aphrodite which had lingered into the Roman Calendar.
May was named for the Goddess Maia, "She who is great." Lots of mag words come from this name. Maximum, magistrate, maestro, and mister are all drawn from her name.
June was named for Juno, queen of the Gods, and patroness of new brides, apparently June has always been the month for marriage. The last six months were simply named numerically, Quintillis (Latin for 5), Sextillia (Latin for six), and then to the ones whose names remain today, September (seventh), October (eighth), November (ninth), and December (tenth).
Around 690 BC Numa Pompilius converted the period of Festival at the end of the year into a month of its own, named for the festival of Februa. This was a period of purification and fertility rituals, that became our present February. Later he added another month to begin the year and named it January after Janus, the God of beginnings and endings. At this point February came at the end of a year, and was followed by January then followed by March to start the next year.
Sometime later, in another adjustment, they switched the order of January and February and adjusted the numbers of days for the months. Adjusting the calendar became a common and very political process under the Roman Senate. By the time of Julias Caesar, there were three full months between the civil equinox and the astronomical equinox. It was this difference that led to his revision of the calendar. At this time (about 44 bc) Julius Caesar also moved the beginning of the year to January. Some argue that Julius himself changed the name of Quintillis (now the seventh month) to July, others say it was done after his death as an honor. Later the month of Sextilla was also renamed, this time for the Emperor Augustus. In the year 1582 Pope Gregory took a shot at calendar rigging. After that most western nations celebrated January first as the start of the year, but in England and her American colonies the new year would begin on March 25, the supposed spring equinox. It was not until 1752 that the British changed the beginning date of the year to January first. There are several other calendar methods still in use in the world. The Hebrew, Chinese, Muslim, and Hindu calendars are used in various parts of the world, and astronomers often use a calendar with a 7980 year period called the Julian Day calendar.