The rule
- Weeks start on Monday.
- Week 1 is the week that contains the year's first Thursday — which is the same as saying the week that contains 4 January.
- Every week belongs wholly to one year, so the first few days of January can belong to the last week of the year before, and the last days of December to week 1 of the next.
So 1 January 2026 fell in week 1 of 2026, and 1 January 2027 falls in week 53 of 2026.
Why some years have 53 weeks
364 days make exactly 52 weeks, so a year has a day or two to spare. Every five or six years they add up to an extra week. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.
Between 2020 and 2050 that happens in 2020, 2026, 2032, 2037, 2043 and 2048.
Other ways of counting
Not everyone uses ISO weeks. In the United States and Canada, calendars often start the week on Sunday and count the week containing 1 January as week 1 — so the same date can have a different week number. Spreadsheets offer both (in Excel, ISOWEEKNUM gives the ISO number, while WEEKNUM uses the US system by default). When someone gives you a week number, it is worth checking which system they mean.