There are seven months in the Gregorian calendar that have 31 days: January, March, May, July, August, October, and December.
The origins of the 31-day month are rooted in the ancient Roman calendar, which was the basis for the Julian calendar created by Julius Caesar in 45 BCE. In the original Roman calendar, months alternated between 29 and 31 days, with the exception of February, which had 28 days. This made for a total of 355 days in a year, which was approximately 10.8 days shorter than the solar year. As a result, the calendar would gradually become misaligned with the seasons.
To address this problem, Caesar reformed the calendar by adding days to certain months to make them longer. He added a day to February every fourth year (leap year), which made it 29 days long. He also added a day to the months of January, Sextilis (which was later renamed August in honor of Augustus Caesar), and December, which made them 31 days long.
The Julian calendar was in use for centuries and remained the standard in Europe until the adoption of the Gregorian calendar in the late 16th century. The Gregorian calendar was introduced by Pope Gregory XIII in 1582 to correct the inaccuracies in the Julian calendar. One of the changes made to the calendar was the addition of a leap year rule, which eliminated leap years in years that are divisible by 100 but not by 400. This change improved the accuracy of the calendar and reduced the number of days in a year from 365.25 to 365.2425.
In the Gregorian calendar, the months with 31 days were retained from the Julian calendar, with the exception of Sextilis, which was renamed August and given 31 days in honor of Augustus Caesar. The other months with 31 days were kept as they were in the Julian calendar.
In summary, the seven months with 31 days in the Gregorian calendar have their origins in the ancient Roman calendar, which was later reformed by Julius Caesar to address the misalignment with the seasons. These months were retained in the Gregorian calendar, which replaced the Julian calendar in the late 16th century.
Table with the months and their number of days
Table with the months of the year and the number of days in each of the months.
| MONTH | NUMBER OF DAYS | LEAP YEAR |
| JANUARY | 31 DAYS | 31 DAYS |
| FEBRUARY | 28 DAYS | 29 DAYS |
| MARCH | 31 DAYS | 31 DAYS |
| APRIL | 30 DAYS | 30 DAYS |
| MAY | 31 DAYS | 31 DAYS |
| JUNE | 30 DAYS | 30 DAYS |
| JULY | 31 DAYS | 31 DAYS |
| AUGUST | 31 DAYS | 31 DAYS |
| SEPTEMBER | 30 DAYS | 30 DAYS |
| OCTOBER | 31 DAYS | 31 DAYS |
| NOVEMBER | 30 DAYS | 30 DAYS |
| DECEMBER | 31 DAYS | 31 DAYS |
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