Short answer
How many weeks are in a year?
Check whether an ISO week-year contains 52 or 53 calendar weeks.
Year overview
Short answer
The week containing 28 December is always the final ISO week and reveals the total.
How it works
A year has 53 weeks only when its weekday pattern creates one extra complete ISO week.
The 53-week rule
An ISO year has 53 weeks when 1 January is Thursday, or when it is a leap year starting on Wednesday. Otherwise it has 52. The ISO week containing 28 December is always the last one of its week-year.
A calendar year has 365 or 366 days: 52 weeks plus one or two days. An ISO week-year consists of complete weeks and is a different period. 2026 has 53 ISO weeks; 2027 has 52, with week 1 starting on 4 January. Neither count determines working days.