Why does 1 January sometimes fall in week 53 of the previous year?

ISO 8601 assigns week 1 to the week containing the first Thursday. When 1 January is a Friday, Saturday, or Sunday, most of that week sits in the previous year, so it stays part of the previous year's final week. That is also why the week year and calendar year can differ.

How many weeks are in a year?

An ISO year has 52 or 53 weeks. It has 53 when 1 January is a Thursday, or when it is a Wednesday in a leap year; otherwise 52.

DATE AND TIME · CALCULATED IN YOUR BROWSER

ISO Week Number Calculator

Find the ISO week for a date, or the dates covered by a given week.

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About this tool

Enter a date to get its ISO week number and week year, or work backwards from a year and week number to the Monday-to-Sunday range it covers. ISO 8601 starts weeks on Monday and defines week 1 as the week containing the first Thursday of the year, which is why 1 January can belong to week 52 or 53 of the previous year and why the week year is not always the calendar year.

How it works

Enter a date, or switch to looking up a week number.
Read the week year, week number, and date range.
Check whether that year has 52 or 53 weeks.

Assumptions

Follows ISO 8601: weeks start on Monday, and week 1 is the week containing the year's first Thursday.

Privacy

Every calculation runs inside this browser. Dates and birthdays are never uploaded, stored, or written to the URL or any log.

Frequently asked questions

Why does 1 January sometimes fall in week 53 of the previous year?: ISO 8601 assigns week 1 to the week containing the first Thursday. When 1 January is a Friday, Saturday, or Sunday, most of that week sits in the previous year, so it stays part of the previous year's final week. That is also why the week year and calendar year can differ.
How many weeks are in a year?: An ISO year has 52 or 53 weeks. It has 53 when 1 January is a Thursday, or when it is a Wednesday in a leap year; otherwise 52.

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