What Week Is It?
What week is it?
Today is Sunday, October 4, 2026, which falls in ISO week 40 of 2026. That week runs from Monday September 28 to Sunday October 4, 2026. Enter any date to find its week number, or use the year pages to see all the weeks laid out.
How ISO weeks are counted
ISO 8601 weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. That rule means that early January can belong to the previous year's last week and late December can belong to week 1 of the next year.
Who uses week numbers
Manufacturers, schools, logistics companies and project managers in much of the world plan by week number. Many calendars in Europe print them, and payroll and production schedules often refer to week numbers rather than dates.
Week numbers by year
Each year page lists how many ISO weeks the year has, which is 52 in most years and 53 in some, and the date range of any week you choose.
Frequently asked questions
What week of the year is it?
It is ISO week 40.
How many weeks are in a year?
52 in most years, 53 in some ISO years.
When does week 1 start?
Week 1 contains the first Thursday of the year.
Which day does the week start?
ISO weeks start on Monday.
Can I look up a date range for a week?
Yes, use the week-number pages.
Is week numbering the same in the US?
Not always. US calendars often start weeks on Sunday and use a different week 1 rule.
Why does December show week 1?
Late December days can belong to week 1 of the following ISO year.
Can I find the week number of a birthday?
Yes, enter the date.