About week numbers
Week numbers give every week of the year a number from 1 to 52 or 53. Businesses use them for planning and reporting, so "week 34" is often clearer than a pair of dates.
How the numbering works
- Weeks run Monday to Sunday.
- Week 1 is the week containing the first Thursday of January.
- Equivalently: the week containing 4 January.
- Most years have 52 weeks; some have 53.
Why can 1 January fall in week 52 or 53?
Because a week belongs to whichever year holds most of it. If 1 January is a Friday, Saturday or Sunday, that week has more days in the old year, so it keeps the old year's last number.
Is this the same everywhere?
This is the ISO 8601 standard, used across Europe and in most business software. Some American systems start weeks on Sunday and number them differently, so check which one your workplace means.
Which years have 53 weeks?
Years starting on a Thursday, and leap years starting on a Wednesday. It works out to roughly once every five or six years.