How Long Is a Common Year?
A common year is a standard Gregorian calendar year with 365 days divided into 12 months, and only 28 days in February.
A Calendar Year
A Gregorian common year consists of 52 weeks and one day. Because of this, January 1 and December 31 of a common year are always on the same day of the week, and the next year begins on the next day of the week. For instance, the common year 2023 starts and ends on a Sunday, and 2024 begins on a Monday.
Out of every 400 years in the Gregorian calendar, 303 years are common years. The rest, 97 years, have an intercalary day or leap year day, making them 366 days long.
Common Years in Other Calendars
The Hebrew Calendar, which is a lunisolar calendar, has 12 months in a common year, which can have anywhere from 353 to 355 days. Leap years in such a calendar have 383 to 384 days and 13 months.
A common year in the Chinese Calendar, which is also a lunisolar calendar, has 12 months with 353—355 days. A month in a Chinese common year corresponds to the cycle of the Moon, from full Moon to full Moon. Because of this, a regular month can have 29 or 30 days. Leap years in the Chinese Calendar have 13 months.
Common years in the Islamic Calendar, which is purely a lunar calendar, have 354 days divided into 12 months. Some months have 30 days while others have 29 days. 11 out of 30 years are Islamic leap years, where the leap year has 30 days in the last month of the year, making the Islamic leap year 355 days. Different communities follow different rules about how to space the leap years in the 30-year leap cycle.
Both solar and lunisolar calendars with regional variations are used in India and the Indian subcontinent. In the solar calendar, the common years have 365 days, with 10 leap years of 366 days each in a cycle of 39 years. Unlike the Gregorian calendar, the Indian solar calendar, also known as the Nirayana, there is no fixed rule as to which year in the cycle will be a leap year.
Two kinds of lunisolar calendars are used in the Indian system, the Amanta in which the months run from new Moon to new Moon, and Purimanta, in which the month runs from full Moon to the next full Moon.
The common year in the Amanta Calendar is 12 months long and is on average 29 days long. The year has 354 days. Seven intercalary months, called Adhik Mass are added at different intervals to the year in a 19-year cycle.