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What week is it?

Today is filled in. Pick any other date to check its week.

Many offices, schools and factories plan by week number. Most use the ISO 8601 system.

How ISO week numbers work

  • Weeks start on Monday.
  • Week 1 is the week with the year’s first Thursday in it (so it always contains January 4).
  • A year has 52 or 53 weeks. Late December can belong to week 1 of the next year, and early January to week 52 or 53 of the last.

Some US calendars count weeks from Sunday with January 1 always in week 1, so their numbers can differ by one.

Frequently asked questions

How many weeks are in a year?

52, or 53 in some years under ISO 8601.

Does the week start on Sunday or Monday?

ISO weeks start on Monday.

Why is January 1 in week 53?

If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.