monthlengths

The lengths of months in the Gregorian calendar are not uniform, contributing to the varying number of days in a year. Most months have either 30 or 31 days. Four months have 30 days: April, June, September, and November. Seven months have 31 days: January, March, May, July, August, October, and December.

The exception is February, which has 28 days in a common year and 29 days in a leap year. Leap years occur every four years, with the exception of years divisible by 100 but not by 400. This system ensures that the calendar year stays aligned with the astronomical year, preventing the seasons from drifting over time. The irregular lengths of months are a historical artifact dating back to ancient Roman calendars, which underwent several revisions before arriving at the current structure. Understanding month lengths is crucial for scheduling, financial calculations, and historical record-keeping.