How many weeks are in a year?

Fifty-two, plus change. A standard year of 365 days divided by 7 gives 52 weeks and 1 day left over; a leap year leaves 2 days. That remainder is the small gear turning the calendar: it is why any given date lands one weekday later than last year, two after a leap day passes, and why calendars only repeat their exact layout every several years.

Payroll speaks this language fluently: 52 weekly paychecks, 26 biweekly, 24 semi-monthly, and the biweekly crowd gets two months a year with a bonus third check, a quirk of the leftover day compounding.

The ISO week-numbering standard used in international business even allows some years 53 numbered weeks, which periodically surprises accounting departments. And the reason for all this arithmetic residue is simply that the Earth's orbit, about 365.2422 days, refuses to be divisible by 7, an inconvenience astronomers have apologized for since the Romans.