There are four months in the Gregorian calendar that have 30 days: April, June, September, and November.
The origins of the 30-day month can be traced back to the ancient Roman calendar, which had months of 29 or 31 days. However, there was a need for a month with a length of 30 days to divide the year into shorter periods of time. In the Roman calendar, the month of February had an uncertain length and was used to balance the number of days in a year.
The Julian calendar, introduced by Julius Caesar in 45 BCE, reformed the Roman calendar and established a standard length of 30 or 31 days for most months. The months of April, June, September, and November were given 30 days, while the remaining months had 31 days, except for February, which had 28 days with an extra day added every fourth year in a leap year.
The Julian calendar remained in use until the late 16th century when the Gregorian calendar was introduced by Pope Gregory XIII in 1582. The Gregorian calendar improved on the Julian calendar by making adjustments to the leap year system to more accurately reflect the length of a solar year. The rule for leap years was revised to exclude years that are divisible by 100 but not by 400, meaning that the year 2000 was a leap year, but the year 1900 was not.
The months with 30 days were retained in the Gregorian calendar, and their lengths were not changed. The months of April, June, September, and November continued to have 30 days, and February continued to have 28 days with an extra day added in leap years.
In summary, the months with 30 days in the Gregorian calendar have their origins in the ancient Roman calendar, which established the need for a month of 30 days to divide the year into shorter periods of time. The Julian calendar, which was introduced by Julius Caesar in 45 BCE, gave the months of April, June, September, and November 30 days, and the Gregorian calendar, which replaced the Julian calendar in 1582, retained the same lengths for these months.
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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