Find the ISO week number (YYYY-Www) for any date, the calendar weekly launch boards use.
How to use the ISO week finder
- Pick a date.
- Read the ISO week and its Monday-to-Sunday range.
How ISO weeks work
Weeks start on Monday, and week 1 is the week with the year's first Thursday. So the first days of January can belong to the previous year's last week.
Reading the result
The big code, such as 2026-W40, is the ISO year, a W and the week number from 01 to 52 or 53. Below it you see the Monday-to-Sunday dates and the Monday's date. The right panel works the other way: type a week code and get its date range. The list underneath shows this week and the next seven, which is handy for planning which week to launch in. Note that the year in the code is the ISO year, which can differ from the calendar year for a few days around New Year.
Where people trip up
Dates from 29 to 31 December can fall in week 1 of the next ISO year, and 1 to 3 January can fall in week 52 or 53 of the previous one. Some years have 53 weeks: those that start on a Thursday, and leap years that start on a Wednesday. US calendars often start weeks on Sunday, and Excel's WEEKNUM uses a different system unless you use ISOWEEKNUM. When a launch board says a week, check the Monday date here rather than counting by hand.