ISO Week Numbers 2026 and 2027: Dates and How Week 1 Is Decided

Week numbers are common in offices, factories, and logistics, especially in Europe, and they show up on many printed calendars as a small number at the start of each row. They look simple, but the rule that decides which week is "week 1" surprises people every January. This guide explains the standard system, ISO 8601, and lists every week of 2026 and 2027 with its dates.

The basic rules

• Weeks start on Monday and end on Sunday.
• Every week belongs entirely to one year. Weeks are never split.
• Week 1 is the week that contains the first Thursday of the year.

The third rule is the one that matters. Another way to say it: week 1 is the week that contains January 4, or the first week in which at least four days fall in the new year.

Why Thursday?

Thursday is the middle day of a Monday-to-Sunday week. If Thursday falls in the new year, then at least four of that week's seven days do too, so it is fair to count the whole week as part of the new year. If Thursday is still in December, most of the week belongs to the old year.

The dates that surprise people

Because weeks are never split, the first few days of January can belong to the last week of the previous year, and the last few days of December can belong to week 1 of the next year.

For example, 2027 starts on a Friday. The week that contains January 1, 2027 runs from Monday, December 28, 2026 to Sunday, January 3, 2027. Its Thursday is December 31, so the whole week is counted as the last week of 2026, which is week 53. Week 1 of 2027 starts on Monday, January 4.

This is why software, spreadsheets, and planners sometimes disagree about the week number in late December or early January. Most of the time, one of them is using a different rule.

ISO week numbers 2026

2026 has 53 ISO weeks. Week 1 runs from Mon, Dec 29, 2025 to Sun, Jan 4, 2026, and week 53 ends on Sun, Jan 3, 2027.

ISO week numbers 2027

2027 has 52 ISO weeks. Week 1 runs from Mon, Jan 4, 2027 to Sun, Jan 10, 2027, and week 52 ends on Sun, Jan 2, 2028.

Why some years have 53 weeks

A year has 52 weeks and one or two extra days. Those extra days add up, so roughly every five or six years there is room for a 53rd week. Under ISO rules, a year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 starts on a Thursday, so it has 53 ISO weeks; 2027 starts on a Friday and has 52. The facts box on each of our 2026 calendars lists the week numbers for that month.

How people use week numbers

• Offices and project teams use them as short references: "delivery in week 22" is shorter than a date range and does not depend on the month.
• Factories and warehouses use them for production and shipping schedules, often printed on product labels.

Week numbers and Sunday-start calendars

ISO week numbers assume a Monday start. On a Sunday-first calendar, each row runs from Sunday to Saturday, so the Sunday at the start of a row actually belongs to the previous ISO week. The preview grid on our calendar pages shows week numbers for reference, while the printed PDF leaves them out. If you work with week numbers every day, a Monday-start calendar lines up better. See Monday or Sunday? for more on choosing.

Short answer

Week 1 is the week with the year's first Thursday in it. Weeks run Monday to Sunday and never split across years, which is why early January can still be week 52 or 53 of the previous year.