Topic
How many weeks are in a year?
A common year has 365 days, which equals 52 weeks plus 1 day; a leap year has 366 days, which equals 52 weeks plus 2 days.
At a glance
- Common year
- 365 days = 52 weeks + 1 day
- Leap year
- 366 days = 52 weeks + 2 days
Overview
Dividing the length of the civil year by seven explains why a calendar year is not an exact whole number of weeks. This is also why the weekday for a date normally shifts by one day the next year and by two after a leap day has occurred. Week-numbering systems such as ISO week dates add another layer because some years contain a week numbered 53.
Calendar years and week-numbering years are different ideas
A civil year begins January 1 and ends December 31. ISO week numbering assigns whole weeks according to its own rules, so the first or last ISO week of a numbered year can include dates from the neighboring civil year.
Why some calendars show week 53
Because 365 and 366 are not divisible by seven, weekday alignment shifts over time. Under ISO week-date rules, certain years therefore contain 53 numbered weeks even though every civil year still contains only 365 or 366 calendar days.
Sources and review
MOOR's explanatory text is supported by the following source links.
- Calendars — NIST
- ISO 8601 Date and time format — International Organization for Standardization