Why Don’t the Months of the Year Have the Same Number of Days?
To solve this mystery, we must dig deep into the history of our modern calendar, which is known as the Gregorian calendar. The Gregorian calendar was a modification of the Julian calendar, which itself was a modification of the ancient Roman calendar.
The ancient Romans, like ancient civilizations before them, based their concept of the month on the Moon. Unfortunately, the lunar cycle is approximately 29.5 days, which does not divide evenly into the 365.25 days that make up a year.
As a result, the earliest ancient Roman calendars had months that were either 29 or 30 days. To make things even more confusing, the ancient Romans borrowed from the ancient Greeks to develop the idea of a 10-month calendar that left approximately 60 days unaccounted for.
For example, the ancient Romans started using a 10-month calendar in 738 B.C. that included the following months: Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. The names Quintilis through December derived from the Latin words for five through ten.
To account for the remaining 60 or so days, Januarius was added to the beginning of the year and Februarius to the end of the year during Numa’s reign around 700 B.C. The calendar stayed in that order until 452 B.C. when a small council of Romans, called the Decemvirs, moved February to follow January.
Julius Caesar modified the Roman calendar in 46 B.C. to make each month have either 30 or 31 days, with the exception of Februarius, which had 29 days and gained an extra day every fourth year.
Quintilis was later renamed Julius in his honor. Likewise, Sextilis later became Augustus to honor Augustus Caesar. Augustus was also given an extra day (taken away from Februarius), so that Augustus and Julius would have an equal number of days.