View ISO week numbers for any year with holiday highlights and quarter boundaries.
This tool renders a full year as a grid of ISO 8601 week numbers, with quarter boundaries marked (Q1 through Q4 by calendar date) and holiday dates highlighted on whichever week they fall in. Enter a year to see the whole grid, or a single date to see which week it belongs to. The recurring surprise: the first days of January and the last days of December often carry a week number from the neighboring year, because ISO weeks always start on Monday and week 1 is defined by the year's first Thursday, not by January 1st.
A concrete input and expected output from the current implementation.
Input
2027-01-01
Expected output
ISO week 53 of 2026 (Monday, Dec 28, 2026 - Sunday, Jan 3, 2027)
January 1, 2027 falls on a Friday, before the Monday (Jan 4) that starts ISO week 1 of 2027, so it is assigned to the last week of the prior ISO week-year; 2026 has 53 weeks because January 1, 2026 fell on a Thursday.
Weeks always run Monday through Sunday. Week 1 of a year is the week containing that year's first Thursday, equivalently the week containing January 4th. Because of this, up to three days at the start of January can belong to the final week (52 or 53) of the previous year, and up to three days at the end of December can belong to week 1 of the next year.
Each week row is annotated with any holiday date that falls inside it, and the grid marks where Q1 through Q4 begin using calendar-quarter boundaries (Jan 1, Apr 1, Jul 1, Oct 1). Because quarters follow the calendar rather than the week grid, a single ISO week can straddle two quarters near each boundary.
Because ISO 8601 defines week 1 as the week holding the year's first Thursday. If January 1 falls on a Friday, Saturday, or Sunday, those first few days still belong to the previous year's final week, not week 1 of the new year.
No. This tool follows the ISO 8601 standard, where every week starts Monday and ends Sunday. If your organization counts weeks starting Sunday, the week numbers shown here will not line up with that system.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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