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About ISO week numbers
October 23, 2026 is in ISO week 43 of 2026. Some years, including 2026, have 53 ISO weeks. Around New Year, a date can belong to the previous or next year's week numbering (for example, January 1, 2021 was in week 53 of 2020).
ISO 8601 rules
ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of January, which is always the week containing January 4. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks.
US-style week numbers
Many US calendars start weeks on Sunday and call the week containing January 1 week 1, so their numbers can differ from ISO by one. In Excel, ISOWEEKNUM gives the ISO week, while WEEKNUM uses the US convention by default.
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Frequently asked questions
Why does January 1 sometimes show week 53?
Under ISO 8601, week 1 is the week with the year's first Thursday, so early January days can belong to the last week of the previous year.
Why does January 1 sometimes fall in week 52 or 53?
If January 1 is a Friday, Saturday or Sunday, that week belongs to the previous ISO year.
Which week number system does Europe use?
Most of Europe uses ISO 8601 weeks starting Monday.