How Many Days in a Month

Seven months have 31 days, four have 30, and February has 28, or 29 in a leap year. For eleven of the twelve months that never changes from year to year. A year of 365 days is 7 x 31 + 4 x 30 + 28, and a leap year adds the one day to February.

The part that does change is what is inside the month. Four weeks is 28 days, so a 31-day month is four weeks and three days, and those three spare days decide which weekdays occur five times. August 2026 starts on a Saturday, so it has five Saturdays, five Sundays and five Mondays, and only 21 weekdays against the 22 or 23 a 31-day month usually carries. That is why a monthly payroll, a rent proration or a "working days this month" figure moves from month to month even when the day count does not. A 30-day month has two weekdays that occur five times, a 29-day February has one, and a 28-day February has exactly four of everything, the only month that can.

Leap years follow the Gregorian rule, introduced in 1582 to correct the drift in the older Julian calendar. A year is a leap year if it is divisible by 4, except that a century year (one divisible by 100) is a leap year only if it is also divisible by 400. So 2024 and 2028 are leap years, 1900 was not, 2000 was, and 2100 will not be. The rule exists because the solar year is about 365.2422 days rather than 365.25; leaping every fourth year overshoots by roughly three days every four centuries, and dropping three of the four century leap days brings the calendar's average year to 365.2425 days.

The Gregorian calendar began in October 1582 and was adopted country by country over the following three centuries (Britain and its colonies switched in September 1752), so before 1583 how many days a month had depends on where you were.