How was it determined how many days each month would receive in our solar calendar?

by ElpredePrime

Apologies for the confusing title. I'm curious how the number of days per month was set in our current Gregorian Calendar?

Every other month has 31 days, except for some consecutive months such as July/August and December/January while February has 28/29 days?

QuickSpore

The short answer is we don't know.

The origins of the Roman calendar are shrouded in myth. The original calendar promulgated by the mythical founder or Rome, Romulus supposedly looked like this:

  • Martius (31 days)
  • Aprilis (30 days)
  • Maius (31 days)
  • Iunius (30 days)
  • Quintilis (31 days)
  • Sextilis (30 days)
  • September (30 days)
  • October (31 days)
  • November (30 days)
  • December (30 days)

You may note that it only has 304 days. It supposedly had an intercalary period between each year. So Martius (March) 1st would always happen at the same time each year as decided by astronomers.

This was reformed by the (also probably mythical) second king of Rome, Numa. According to Macrobius and Plutarch it looked like this:

  • Ianuarius (29 days)
  • Februarius (28 days)
  • Martius (31 days)
  • Aprilis (29 days)
  • Maius (31 days)
  • Iunius (29 days)
  • Quintilis (31 days)
  • Sextilis (29 days)
  • September (29 days)
  • October (31 days)
  • November (29 days)
  • December (29 days)

The months that had 31 days were preserved. Every few years a 27 day intercalary month would be inserted as deemed necessary to realign the calendar to the seasons. But the system for that wasn't regular. Decades could pass without a leap year, or you could have them back to back, according to the whims of the pontiffs.

You'll notice a distinct lack of explanation for why there were short months and long months, or why February was extra short. No one seemed to know or care. Regardless of origins, we do know this was the system in use for most of the Republic.

When Julius Caesar made his Julian calendar in 46 BCE he preserved the long month / short month differentiation, although he added a day to the short months to bring them up to 30. He also added an extra day to December for some reason, and no, we don't know why. The legend that Augustus stole a day from March for August is just that, a legend. February had always been extra short. Caesar also instituted the leap day in February. This gave us the basis for the modern Georgian calendar.

I can tell you when the various changes happened. But what is annoyingly absent is why. The answer to why March has 31 days or why February has 28, apparently is that the mythological figures Romulus and Numa decided it should be so... and so it has been for the last 2,700 years, as unsatisfactory as that answer may be.