2040 has 52 ISO weeks: week 1 runs from Monday, January 2 to Sunday, January 8, 2040, and week 52 ends on Sunday, December 30, 2040. January 1, 2040 belongs to week 52 of 2039. December 31, 2040 belongs to week 1 of 2041.
Weeks start on Monday. Every ISO week runs Monday to Sunday and belongs to exactly one year.
Week 1 contains the first Thursday. Equivalently, it is the week with January 4, or the first week with at least four days in the new year. In 2040 the first Thursday is January 5.
Year boundaries. Because week 1 must contain a Thursday, December 29–31 can fall in week 1 of the next year, and January 1–3 can fall in week 52 or 53 of the previous year. January 1, 2040 belongs to week 52 of 2039. December 31, 2040 belongs to week 1 of 2041.
US week numbers in 2040 (Sunday start)
Many US wall calendars, and the WEEKNUM() function in Excel and Google Sheets by default, number weeks differently: weeks run Sunday to Saturday, and week 1 is the week that contains January 1, however short. Counted that way, 2040 has 53 weeks; week 1 is Sunday, January 1 – Saturday, January 7.
Between January 2 and December 30, 2040, Monday–Saturday dates have the same week number in both systems, and Sundays are one week higher in the US count, because the US week starts a day earlier. The holiday table above shows both numbers for each federal holiday.
Frequently asked questions
Which years have 53 ISO weeks?
Years that start on a Thursday, and leap years that start on a Wednesday; all others have 52. From 2020 to 2040 these are 2020, 2026, 2032 and 2037. 2040 starts on a Sunday, so it does not.
How is a date written as an ISO week date?
As the ISO year, the letter W with the two-digit week, and the weekday number from 1 (Monday) to 7 (Sunday). For example, May 1, 2040 is written 2040-W18-2: the Tuesday of week 18.
How do I get the ISO week number in Excel or Google Sheets?
Use =ISOWEEKNUM(date) (Excel 2013 and later, Google Sheets) or =WEEKNUM(date,21). For example, =ISOWEEKNUM(DATE(2040,1,8)) returns 1 for Sunday, January 8, 2040, while the default =WEEKNUM(DATE(2040,1,8)), which counts Sunday-start weeks, returns 2.