Calendar weeks: the underrated ordering system
"Delivery in week 42", "holiday in weeks 31 and 32", "deadline end of week 9": in German working life the calendar week — the "KW" — is everywhere. It is shorter than a date, more precise than "mid-October" and ideal for project plans, shift rotas and supply chains. Yet few people know the rule by which the weeks are actually counted — or why 1 January sometimes falls in week 53.
You can look up the current week, the Monday and Sunday of any week and the yearly overview with the public holidays of each German state at any time in the calendar week calculator. This guide explains what lies behind it.
The basic rule: ISO 8601
Germany, Austria, Switzerland and most European countries count calendar weeks according to the international standard ISO 8601, adopted in Germany as DIN EN ISO 8601. The rule was already part of the older German standard DIN 1355. It consists of two simple definitions:
1. A week always starts on Monday and ends on Sunday.
2. The first calendar week of a year is the week that contains the first Thursday of the year.
The second rule gives a handy shortcut: 4 January is always in week 1. If 4 January lies in a week, that week also contains at least one Thursday of the new year. So to find the start of week 1, simply look for the Monday on or before 4 January.
Why Thursday?
Thursday is the middle day of a Monday-to-Sunday week. If it falls in the new year, at least four of the seven days of that week belong to the new year — the majority. The standard therefore always assigns a week to the year in which most of its days fall. This is fair and unambiguous: no week is split, each belongs to exactly one year.
The price for this is that weeks do not fit neatly into the calendar year. The first days of January can still belong to the last week of the previous year, and the last days of December can already belong to the first week of the next year.
Examples around the turn of the year
A current example shows how this works in practice. 1 January 2026 is a Thursday. The week from Monday, 29 December 2025, to Sunday, 4 January 2026, therefore contains the first Thursday of 2026 and is week 1 of 2026. The 29th, 30th and 31st of December 2025 already belong to week 1 — of 2026.
At the other end of the year the opposite happens. 1 January 2027 is a Friday. The week from 28 December 2026 to 3 January 2027 contains Thursday, 31 December 2026, and therefore no Thursday of 2027. It is thus week 53 of 2026. Week 1 of 2027 only begins on Monday, 4 January 2027.
Remember: every calendar week actually has its own "week-numbering year". It is usually identical to the calendar year and only differs in the days around New Year. The calculator therefore shows you, for every date, which week-numbering year the week belongs to.
52 or 53 calendar weeks?
A year has 365 or 366 days — 52 weeks plus one or two days. These surplus days add up, and roughly every five to six years an additional week appears: week 53. This happens exactly when a year starts on a Thursday or, in a leap year, on a Wednesday. In a 400-year calendar cycle this occurs 71 times.
The years with 53 calendar weeks this century include 2004, 2009, 2015, 2020, 2026, 2032, 2037, 2043 and 2048. Why 2026 is one of them and what this means for weekly schedules is explained in Why 2026 has 53 calendar weeks.
How to work out the calendar week yourself
If you want to do without a calculator, you can determine the week of a date with a small trick:
1. Find the Thursday of the same week (Monday to Sunday).
2. The year of that Thursday is the week-numbering year.
3. Work out which day of the year this Thursday is and divide by seven. Rounded up, this gives the calendar week.
Example: 24 September 2026 is itself a Thursday and the 267th day of the year. 267 divided by 7 is 38.1 — rounded up, week 39. Conversely, you find the Monday of a given week by counting forward from the Monday of week 1 (the Monday on or before 4 January) by the desired number minus one, times seven days.
In everyday life that is cumbersome, which is why the calendar week calculator handles both directions instantly: date in, week out — or week and year in, Monday and Sunday out.
Watch out with international appointments
Not everyone counts according to ISO 8601. In the USA, Canada and some other countries the week traditionally starts on Sunday, and the first week is simply the week containing 1 January. The same week can therefore carry a number that differs by one, especially at the start of the year. Some spreadsheet programs and calendar apps also count this way by default and need a dedicated ISO week function or a changed setting.
If you work with partners abroad, always state a specific date for important deadlines, or explicitly write "ISO 8601 week". The standard even has its own notation: 2026-W39 stands for week 39 of 2026, and 2026-W39-4 for its Thursday.
Calendar weeks and public holidays
For planning, calendar weeks are most useful when you can see at a glance which weeks are shortened. Germany has nine nationwide public holidays; the federal states add more. Corpus Christi and All Saints' Day, for example, only apply in some predominantly Catholic states, Reformation Day in the eastern and northern states, the Day of Repentance and Prayer only in Saxony, World Children's Day in Thuringia and International Women's Day in Berlin and Mecklenburg-Western Pomerania.
The movable holidays depend on the date of Easter: Good Friday is two days before Easter Sunday, Ascension Day 39 days after it, Whit Monday 50 and Corpus Christi 60 days after it. In 2026, Easter Sunday falls on 5 April. The calculator computes these dates for every year and also shows in the yearly overview how many working days each week and month has in your state. If you are planning your leave, you can continue straight away in the holiday entitlement calculator.
Conclusion
The calendar week follows two clear rules: Monday is the first day, and week 1 is the week with the first Thursday. All the peculiarities follow from this — weeks that span the turn of the year, and years with 53 instead of 52 weeks. Knowing this logic helps you avoid misunderstandings over delivery dates, holiday plans and international arrangements. For a quick look at the current week, any date range and the holidays of your state, use the calendar week calculator.