Common year
A common year in the Gregorian calendar is a standard calendar year consisting of 365 days, divided into 12 months, with February having only 28 days.[1] This contrasts with a leap year, which includes an additional day on February 29, resulting in 366 days total.[2] The Gregorian calendar, introduced in 1582 by Pope Gregory XIII to refine the earlier Julian calendar, approximates the tropical year—Earth's orbit around the Sun—at about 365.2425 days by designating most years divisible by 4 as leap years, except for century years not divisible by 400.[3] Common years form the majority of years in the Gregorian system, comprising 303 out of every 400 years, while leap years account for the remaining 97.[1] A common year spans 52 weeks and 1 extra day, causing the days of the week to shift forward by one from the previous year (or two if the prior year was a leap year).[1] This structure ensures the calendar remains closely aligned with astronomical seasons, preventing gradual drift such as the over-accumulation of days seen in the Julian calendar.[4] The term "common year" distinguishes these standard years from the intercalary leap years, and they are the default for civil, religious, and astronomical purposes worldwide.[5]Fundamentals
Definition
A common year, also known as an ordinary year, in the Gregorian calendar consists of 365 days. This duration equates to exactly 52 weeks and 1 day, as 365 divided by 7 yields 52 full weeks (364 days) with one additional day that advances the weekly cycle.[6] The Gregorian calendar uses common years to approximate the tropical year, the time required for Earth to complete one orbit around the Sun relative to the vernal equinox, which measures approximately 365.2422 days.[7] In common years, no extra day is added, unlike leap years, which insert February 29 to better align the calendar with this solar period.[8] Common years are structured into 12 months with the following fixed day counts: 31 days each in January, March, May, July, August, October, and December; 30 days each in April, June, September, and November; and 28 days in February. This distribution totals 365 days and maintains a consistent annual framework across non-leap periods in the calendar.[7]Distinction from Leap Year
A common year comprises 365 days, in contrast to a leap year, which has 366 days due to the insertion of an intercalary day designated as February 29.[8] This extra day causes the calendar in a leap year to advance by two weekdays over the year, whereas a common year advances by only one weekday.[7] The distinction serves to synchronize the calendar with Earth's orbital period around the Sun, preventing seasonal misalignment. The tropical year—the interval between successive vernal equinoxes—measures approximately 365.2422 days.[7] Each common year thus produces a cumulative drift of roughly 0.2422 days relative to the seasons, which leap years mitigate by adding the extra day roughly every four years.[7] Over a four-year period with three common years and one leap year, the total duration is 1,461 days, equivalent to 208 weeks and 5 days.[7] This configuration yields an average year length of 365.25 days, approximating the tropical year and thereby maintaining long-term calendar-orbital alignment.[7]Gregorian Calendar
Identification Rules
In the Gregorian calendar, a year is classified as a common year if it does not satisfy the criteria for a leap year, resulting in 365 days rather than 366.[2] The primary rule states that a year is a common year if it is not evenly divisible by 4, or if it is divisible by 100 but not by 400.[9] This ensures alignment with the solar year's approximate length of 365.2425 days by occasionally omitting the extra day added in February for leap years.[10] The algorithmic determination of a common year can be expressed through the inverse of the leap year condition: a year is common if it fails the leap year test, which is defined as follows—check if (year modulo 4 equals 0) and (either year modulo 100 does not equal 0, or year modulo 400 equals 0); if this condition is false, the year is common.[2] In pseudocode form:This logic prioritizes the basic divisibility by 4 for most years while applying exceptions for century years to correct for accumulated errors in earlier Julian calendar practices.[9] Century years illustrate key edge cases in this rule. For instance, years such as 1700, 1800, and 1900 are common years despite being divisible by 4, because they are divisible by 100 but not by 400, thus skipping the leap day.[10] In contrast, 1600 and 2000 qualify as leap years since they are divisible by 400, overriding the century exception.[2] These adjustments prevent the calendar from drifting relative to the seasons over centuries.[9]if (year % 4 != 0) || (year % 100 == 0 && year % 400 != 0) { return "common year"; } else { return "leap year"; }if (year % 4 != 0) || (year % 100 == 0 && year % 400 != 0) { return "common year"; } else { return "leap year"; }