How ISO Week 1 Is Defined — The First Thursday Rule
Updated: May 2026
ISO week 1 is not simply the first seven days of January. The ISO 8601 standard uses a precise rule based on Thursday — the middle weekday — to determine which week earns the label "week 1" each year.
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The official rule
ISO 8601 defines week 1 as the week containing the year's first Thursday. Because ISO weeks start on Monday, this week runs from a Monday to a Sunday, and Thursday is its fourth day. The week that contains the first Thursday necessarily has at least four of its seven days in the new year.
Three equivalent formulations of the same rule:
- The week containing the first Thursday of the year.
- The week containing January 4 (January 4 is always in week 1 by construction).
- The first week that has a majority (≥ 4 days) of its days in the new year.
All three formulations always produce exactly the same week. Whichever you find easiest to remember, the result is identical.
Why Thursday is the anchor day
Thursday is the middle of the ISO weekday sequence (Monday = 1, Thursday = 4, Sunday = 7). Using Thursday as the anchor ensures that week 1 always has a majority of days in January. If the anchor were Monday, a week starting December 29 would be called week 1 even though six of its seven days are in December. If the anchor were Sunday, a week ending January 1 could claim to be week 1 even though six days are in the previous year.
By anchoring on Thursday — the median weekday — the rule guarantees that week 1 is "mostly" in the new year, creating a semantically clean boundary.
When does week 1 start? Examples by year
| Year | Jan 1 day | Week 1 starts | Note |
|---|---|---|---|
| 2024 | Monday | Jan 1, 2024 | Jan 1 is Monday — it is the start of W1 |
| 2025 | Wednesday | Dec 30, 2024 | First Thursday falls Jan 2 — W1 starts Dec 30 |
| 2026 | Thursday | Dec 29, 2025 | Jan 1 is Thursday — W1 starts the Monday before: Dec 29 |
| 2027 | Friday | Jan 4, 2027 | Jan 1–3 belong to W53/2026; first Thursday is Jan 7 → W1 starts Jan 4 |
| 2028 | Saturday | Jan 3, 2028 | First Thursday is Jan 6 → W1 starts Jan 3 |
Notice that week 1 of 2026 starts on December 29, 2025. Those last three December days carry the ISO label "2026-W01" even though they are calendar-year 2025 dates.
Days that belong to week 1 of the next year
The flip side of the Thursday rule: the last days of December can belong to week 1 of the following year. Specifically, December 29, 30, and 31 may carry an ISO year that is one higher than the calendar year if they are part of a week whose Thursday falls in January.
This is not an error or an edge case to work around — it is the intended behavior of ISO 8601. Software that processes ISO week dates must handle ISO year separately from calendar year for these boundary dates.
Checking week 1 boundaries with the calculator
The Flowfiles ISO week calculator lets you enter any date and instantly see its ISO year and week number. To confirm the start of week 1 for any given year, enter January 4 of that year — it will always return week 1. You can also use the "Week to date range" section: enter the year and week 1 to get the exact Monday-to-Sunday span.