Why are months so weird?

Calendars throughout history have struggled to be logical and maintain synchronism between the lunar cycle (about 29.5 days) and the solar year (about 365.25 days).

Most systems include a bodge factor or intercalary months or days to bring things back into line, with varying degrees of accuracy.


The ancient Sumerian calendar had 12 months divided into 29 or 30 days with an extra month added periodically.  As there was considerable religious diversity, months were named “first month”, “second month” etc.  There were no weeks.

Ancient Greece

The ancient Greeks also used a 12 month calendar and added an extra month every other year.

The Greek calendar was as follows:

  • Hekatombion,
  • Metageitnion,
  • Boedromion,
  • Pyanepsion,
  • Maimakterion,
  • Poseidon,
  • Gamelion,
  • Anthesterion,
  • Elaphebolion,
  • Munychion,
  • Thargelion,
  • Skirophorion

The extra month was added after Poseidon and was called second Poseidon.

The length of each month (29 or 30 days) was announced at the start of the month to try and synch with the lunar cycles and usually alternated between 29 and 30 days but occasionally there would be two 29 day months in a row.

Ancient Egypt

The ancient Egyptian calendar was split into three seasons, each with four months of 30 days.  Each month was divided into three 10-day periods known as decans or decades.  Workers did not have to work on the last two days of the decans.

An extra month of 5 days was added every year and was not considered as part of the year.  The Egyptian priests refused to add a further day every four years (like a modern leap day) causing the calendar to drift until eventually it was replaced.

  • Akhet Thoth (First Month of Flood)
  • Akhet Phaophi (Second Month of Flood)
  • Akhet Athyr (Third Month of Flood)
  • Akhet Chioak (Fourth Month of Flood)
  • Peret Tybi (First Month of Growth)
  • Peret Mechir (Second Month of Growth)
  • Peret Phamenoth (Third Month of Growth)
  • Peret Pharmuthi (Fourth Month of Growth)
  • Shemu Pachons (First Month of Low Water)
  • Shemu Payni (Second Month of Low Water)
  • Shemu Epiphi (Third Month of Low Water)
  • Shemu Mesore (Fourth Month of Low Water)
  • Extra month


The ancient Roman calendar is believed to have 10 months of 30 or 31 days with 50 days of “unorganised winter”.  It is more likely that months were added to bring things back into synch.

  • March (Month of Mars – 31 days)
  • April (Unknown derivation – 30 days)
  • May (Unknown derivation – 31 days)
  • June (Month of Juno – 30 days)
  • Quintilis (Fifth Month – 31 days)
  • Sextilis (Sixth Month – 30 days)
  • September (Seventh Month – 30 days)
  • October (Eighth Month – 31 days)
  • November (Ninth Month – 30 days)
  • December (Tenth Month – 30 days)
  • Extra Month/Period (50 days)

The calendar of the Roman Republic mostly followed the greek system and added two months of  January and February but did not follow the 29/30 day system.

Instead, they defined 31 days for “lucky” months (relating to superstitions about odd numbers) and 29 days for other months, except February which had 28 days for 3 years and an additional day on the fourth year (leap day).

The intercalary month (extra days) was added in the middle of February.  February was the first month of the year so the numbering remained consistent.

  • January (Month of Janus – 29 days)
  • February (Month of Februa – 28 days)
  • Mercedonius (Month of Wages – intercalary month – 23 days)
  • March (Month of Mars – 31 days)
  • April (Unknown derivation – 29 days)
  • May (Unknown derivation – 31 days)
  • June (Month of Juno – 29 days)
  • Quintilis (Fifth Month – 31 days)
  • Sextilis (Sixth Month – 29 days)
  • September (Seventh Month – 29 days)
  • October (Eighth Month – 31 days)
  • November (Ninth Month – 29 days)
  • December (Tenth Month – 29 days)

There was still some drift, and when Julius Caesar rose to power he ordered a reformation of the calendar (46BC).

To realign the calendar  (in 46BC) he extended the year to 445 days by the addition of two months between November and December and these were known as Intercalaris Prior and Intercalaris Posterior.

10 days were added to the previous calendar of 355 days to create a year of 365 days. Two extra days were added to January, Sextilis and December, and one extra day was added to April, June, September and November.  February remained unchanged as 28 days expect in leap years when it became 29 days.  These lengths continue to be used today.

The additional intercalary day was added five days before the end of February replacing the old Month of Wages intercalary month.

Quintilis was renamed Julius (July) in 44BC and Sextilis as Augustus (August) in 8 BC.

The Julian calendar corrected most of the problems with drift but a 0.002% drift remained.

Gregorian Calendar

The Gregorian calendar, ordered by Pope Gregory XIII to help set the date for Easter,  corrected the small drift inherent in the Julian calendar by adding the following rules:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400.
For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.

This is the calendar that is in common use today.











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