text: The Roman Year used to start on March 1st so the last day in February was New Year's Eve.\n\nMy understanding is that there always were 12 months, as the word is derived from Moon, which rotates around the earth once every 29 days or so. 12 times and a bit in a solar year.And September is simply the 7th month, starting counting from March as the 1st month. \n\nSo I think the idea that there were ten months is jumbling the facts. December was the tenth month but it wasn't the end of the calendar year, January was then the 11th and February the 12th month.\n\nMarch 1st being the year-start makes it kind of logical to add or subtract days at the end of February. When we added a leap second a few years ago, it was added to the end of the year on 31st December. Our current year-end.\n\nAs far as I know, there was only one change of length of month because of honouring an Emperor. August used to be 30 days long. When they added a day to make it 31 and equal the length of July (to imply Augustus was Julius' equal) and named the month August, it was at this point in time that they changed February from 29/30 days to 28/29 days.\n\nSo before Augustus Caesar's time there were people with birthdays called Feb 30th but nobody with a birthday called August 31st... that date simply did not exist in the calendar.\n\nSo if you asked "What day of the week was August 31st in BC 100?" the answer would be that there was no day called August 31st in 100 BC. It is a trick question, like "What was the next day after September 2nd 1752?" (in England) to which the answer is "September 14th 1752!", because that was the year the Julian Calendar was changed to the Gregorian Calendar and 11 days were lost from September that year.