Today is Wednesday, September 30, 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.
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.
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.
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.
Today is Wednesday, September 30, 2026. That is day 273 of 365 in 2026, in ISO week 40, with 92 days to go before the year ends. The next dates people plan around are Diwali on Saturday, October 10; Columbus Day on Monday, October 12; Halloween on Saturday, October 31. The moon is a Waning Gibbous tonight, with about 81% of its face lit. All of these figures are recalculated when the page loads, so they always describe the day you open it, not the day the page was published.
The Gregorian calendar has twelve months of 28 to 31 days. A normal year has 365 days and a leap year has 366, with the extra day added to February in every year divisible by four except century years that are not divisible by 400. Because 365 is one more than 52 weeks, most dates move forward one weekday each year and two after a leap day. Weeks are numbered under ISO 8601, which starts weeks on Monday and gives week 1 to the week containing the first Thursday of January.
It is ISO week 40.
52 in most years, 53 in some ISO years.
Week 1 contains the first Thursday of the year.
ISO weeks start on Monday.
Yes, use the week-number pages.
Not always. US calendars often start weeks on Sunday and use a different week 1 rule.
Late December days can belong to week 1 of the following ISO year.
Yes, enter the date.
Yes.
-04:00.