The Roman calendar had 10 months, starting at the vernal equinox in Martius (March). January and February were added later to the beginning, shifting each month back by 2 months.
The early Roman calendar had 10 months named Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. In that calendar, the last 6 months used the recognized numerical prefixes.
This calendar had a major flaw in that the days in all the months didn’t add up to a full year. A couple of centuries later, January and February were added to the calendar to bring the calendar closer to 365 days.
When these two months were prepended to the calendar it seems that it didn’t occur to them to rename the others (or to add January and February to the end). So now the prefixes don’t match the numerical order.
So, what happened to Quintilis and Sextilis?
In 44BC Quintilis was renamed to July to honour Julius Caesar. Later in 8BC Sextilis was renamed to August to honour Augustus Caesar.
What happened with the other names?
Over the centuries, the Roman calendar was replaced with the Julian calendar, and then finally (for now) the Gregorian calendar. The number of days in each month and the names of the months were tweaked to get to where we are today.
Bonus Biscuit: What is the meaning of all the month names?
- January: named after Janus, Roman god of doors, beginnings and endings (and god of gods).
- February: named for the Roman festival of purification called Februa.
- March (Martius): named after Mars, the Roman god of war.
- April (Aprilis): There are conflicting theories.
- after aprilis in meaning second in Latin
- after aperire, Latin for ‘to open’, suggesting the opening of buds and floweres
- after the Aphrodite, Roman goddess of love and beauty
- May (Maius): named after Maia, Roman goddess of fertility. Some suggest that it was Maia the Greek goddess of the earth, or even maius the Latin word for growth and spring.
- June (Junius): named after the Roman goddess Juno, patroness of marriage and the well-being of women
- August: named after Augustus Caesar
- July: named after Julius Caesar
- September through October: meaning 7th, 8th, 9th and 10th month