Week-number guide
When is there a Week 53?
A normal ISO week-year has 52 weeks. Some years need an additional Week 53 to keep complete Monday-to-Sunday weeks aligned with the calendar.
How can you tell?
December 28 always belongs to the final ISO week of its ISO week-year. If December 28 is in Week 53, that ISO year has 53 weeks.
Why does Week 53 exist?
Fifty-two full weeks contain 364 days, while calendar years contain 365 or 366 days. The ISO Week 1 rule occasionally causes the extra days to produce a 53rd complete numbered week.