that contain 53 Thursdays). An average year is exactly 52.1775 weeks long; months (1/12 year) average at exactly 4.348125 weeks/month.

An ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. These 53-week years occur on all years that have Thursday as 1 January and on leap years that start on Wednesday. The extra week is sometimes referred to as a leap week, although ISO 8601 does not use this term.

Weeks

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