Why Week 1 Can Start in December

A surprise on the calendar

Open a calendar with week numbers and look at the end of December 2025. Monday 29 December 2025 is shown as week 1. The year is not over, the week number has already reset, and it looks like a bug. It is the ISO rule working as designed: the week belongs to the year that contains its Thursday, and the Thursday of that week is 1 January 2026.

The same thing happens at the other end. Friday 1 January 2027 belongs to week 53 of 2026, because its Thursday is 31 December 2026. A calendar year and an ISO year are two different ways of cutting the same days, and they overlap for up to three days at each end.

The rule behind it

Week 1 of a year is the week that contains the first Thursday of January. Equivalent definitions are that it contains 4 January, or that at least four of its days are in the new year. A week always runs from Monday to Sunday, so week 1 begins on the Monday between 29 December and 4 January.

If 1 January is a Monday, Tuesday, Wednesday or Thursday, then week 1 includes 1 January and starts on the Monday of that week, between 29 December and 1 January. If 1 January is a Friday, Saturday or Sunday, then 1 January belongs to the previous year's last week, and week 1 starts on the next Monday, between 2 and 4 January.

Which Mondays in December start week 1?

Week 1 starts on 29 December when 1 January is a Thursday, as in 2026. It starts on 30 December when 1 January is a Wednesday, as in 2025 and 2020. It starts on 31 December when 1 January is a Tuesday, as in 2030. It starts on 1 January itself when 1 January is a Monday, as in 2024 and 2029.

For these years the new ISO year starts before the calendar year does. Week 1 of 2025 runs from Monday 30 December 2024 to Sunday 5 January 2025. Week 1 of 2030 runs from Monday 31 December 2029 to Sunday 6 January 2030. Tuesday 31 December 2024 is therefore in week 1 of 2025, and a schedule written in week numbers treats it that way.

What the first days of January do

The reverse case is a week that starts in December and stays with the old year. In 2027, 1 January is a Friday. It belongs to week 53 of 2026, along with 2 and 3 January. In 2028, 1 January is a Saturday, and it belongs to week 52 of 2027. In 2023, 1 January was a Sunday and belonged to week 52 of 2022.

So there are three groups of January days. Days that are in week 1 of the new year, days that still belong to week 52 or 53 of the old year, and in some years a mix of both in the first few days. The calculator shows both numbers, the week and the week-year, so you can see which group a date is in.

What goes wrong in practice

Reports that group by calendar year and week number get the first and last days wrong. If you take the year from the date and the week number from the ISO rule, then 31 December 2024 becomes "week 1 of 2024", which is a week that ended a year earlier. The row is in the wrong place, or it merges with the real week 1 of 2024. The fix is to use the ISO week-year together with the ISO week number.

Spreadsheet users meet the same problem. A pivot table that groups by year and by ISOWEEKNUM puts the last days of December in the first week of the same year. Adding a helper column for the week-year, built from the Thursday of the week, repairs it.

In software, the right approach is to store dates as dates and calculate week and week-year together, in one function, from a single source. Do not calculate the year separately and do not store a week number as the only record of a date.

What calendar apps show

Most calendar apps with week numbers follow the rule of the region you select. A calendar set to a European region shows 29 December 2025 as week 1, and an app set to the United States may show week 53 or week 1 for the same day. Neither is broken. They count by different rules.

If you share a calendar with people in another country, the safest approach is to agree on the dates, not just the week number. An invitation that says "Week 1, Monday 29 December" cannot be misread.

How to plan around it

For people, the practical advice is simple. If a plan crosses New Year, write the dates next to the week numbers. "Week 1, 29 December to 4 January" is clear. "Week 1" alone invites the question of whether the holiday week is meant.

For payroll and deadlines, check whether the rule is by date or by week. A task due "end of week 1" in 2026 is due on Sunday 4 January 2026, or for most workplaces on Friday 2 January. A task due "in week 1" can fall on 29 December, which is not in January at all. State the date, not only the week, when money or contracts depend on it.