How it is calculated
Under ISO, week 1 of a year is the week that contains the first Thursday. The 1st of January can therefore belong to the last week of the previous year, when it falls on a Friday, a Saturday, or a Sunday.
ISO week: the Thursday of the current week sets the week year
Where the numbers come from
- Date
Example
January 1, 2024 is a Monday and week 1 of 2024.
Questions
Why is January 1 sometimes not week 1?
If the Thursday of this week falls in the previous year, the week belongs to that previous year.
Does the week start on Sunday?
No. This is ISO only, starting on Monday.
How many weeks in all?
52 or 53. The calculator shows the week number of one date, not the length of the year.