Why do some years have 53 weeks?
Quick Answer: Some years have 53 weeks because the calendar weeks are counted based on how the days fall. If a year starts or ends on a Thursday, or if it’s a leap year starting on a Wednesday, it will have an extra week, making 53 weeks instead of the usual 52.
Weeks are counted according to the ISO week date system, where each week starts on Monday and Week 1 is the one containing the first Thursday of the year. This system means most years have 52 weeks, but sometimes the calendar spills over, creating a 53rd week.
That extra week happens when January 1st is on a Thursday, or December 31st falls on a Thursday. In leap years, starting on Wednesday can also cause this. It’s all about how the days line up, not the number of days in the year.