[1] More generally, a common year is one without intercalation.
The Gregorian calendar (like the earlier Julian calendar) employs both common years and leap years to keep the calendar aligned with the tropical year, which does not contain an exact number of days.
[2] The common year of 365 days has 52 weeks and one day, hence a common year always begins and ends on the same day of the week (for example, January 1 and December 31 will fall on a Wednesday in 2025) and the year following a common year will start on the subsequent day of the week.
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