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Does December go to 31?

Yes, December is one of the months that has 31 days. December is the last month of the year in the Gregorian calendar and has 31 days. The other months that also have 31 days are January, March, May, July, August, October, and November. February is the only month with fewer than 31 days, having either 28 or 29 days depending on whether it is a leap year.

Why does December have 31 days?

December has 31 days because of how the Gregorian calendar is structured. The Gregorian calendar, which is the most widely used calendar system today, has 12 months with either 30 or 31 days, except for February which has 28 days or 29 days on leap years.

December comes at the end of the year and was made to have 31 days to help keep the calendar aligned with the solar year. The Gregorian calendar repeats in a 400 year cycle, with 97 leap years and 303 regular years. This pattern provides an average calendar year length that closely matches the actual solar year length of 365.2425 days. Having months alternate between 30 and 31 days, with the 31 day months placed strategically, helps achieve this alignment.

Do all December months have 31 days?

Yes, every December has 31 days in the Gregorian calendar. The number of days in each month was set when the Gregorian calendar was established in 1582. Unless there are future reforms made to the calendar system, December will always have 31 days.

Other calendar systems may have different rules for their December months. For example, in the Julian calendar that preceded the Gregorian, December had 30 days. When the switch was made to the Gregorian calendar in 1582, days were adjusted to realign the calendar, which included changing December to 31 days. But in the Gregorian calendar now used internationally, December consistently has 31 days.

What are the months that have 31 days?

The months that have 31 days are:

  • January – 31 days
  • March – 31 days
  • May – 31 days
  • July – 31 days
  • August – 31 days
  • October – 31 days
  • December – 31 days

So January, March, May, July, August, October, and December all have 31 days. The mnemonic “Thirty days hath September, April, June and November” can help remember which months have 30 days versus 31 days. February is the only exception with 28 or 29 days depending on the year.

Chart Showing Number of Days in Each Month:

Month Number of Days
January 31
February 28 or 29 (leap year)
March 31
April 30
May 31
June 30
July 31
August 31
September 30
October 31
November 30
December 31

When did December start having 31 days?

December started having 31 days when the Gregorian calendar began being implemented in 1582. Before that, the Julian calendar was used which had December with 30 days.

The Gregorian calendar was introduced by Pope Gregory XIII as a reform to the Julian calendar. The Julian calendar had calculated the solar year length incorrectly, so the calendar date was drifting compared to the actual solar seasons. The Gregorian calendar corrected this by more accurately calculating the solar year length and adjusting the leap year rules.

As part of transitioning to the Gregorian calendar, days and months were aligned to put the date back in sync with the seasons. This included changing December from 30 to 31 days. When countries switched to the Gregorian calendar, they accepted December having 31 days as part of adopting this improved calendar system.

Key Dates:

  • 45 BC – Julian calendar created with December having 30 days
  • 1582 – Gregorian calendar introduced with December changed to 31 days
  • 1582-1752 – Adoption of Gregorian calendar spreads across the world

So December has had 31 days since the institution of the Gregorian calendar over 400 years ago.

Does December have 31 days every year?

Yes, December has 31 days in the Gregorian calendar every single year without exception. Unlike February which changes between 28 and 29 days depending on the year, December always has the same number of days.

The Gregorian calendar repeats on a consistent 400 year cycle. Each 400 year cycle has 303 regular years with 365 days and 97 leap years with 366 days. This pattern gives an average calendar year length that closely approximates the true astronomical solar year. Having unchanging 31 day months like December in this calendar design helps achieve the goal of keeping the calendar aligned with the seasons.

So while leap years add an extra day to February, the number of days in December remains fixed at 31. December 31st is the last day of the year, every year without fail in the Gregorian calendar.


In summary, December has 31 days in today’s predominant Gregorian calendar. It was given 31 days when the Gregorian calendar was instituted in 1582 to realign the calendar date with the solar seasons. The months with 31 days occur on a regular pattern within the calendar, helping maintain an average year length that matches the solar year.

December’s 31 days provide consistency as the final month of every year. Unlike February’s variation between 28 and 29 days, December always ends the year with the same 31 day duration. So December 31st is firmly established as New Year’s Eve, the last calendar day before a new year begins on January 1.