ISO Week Numbers 2025
2025 has 52 ISO 8601 weeks. Week 1 runs from 30 Dec 2024 to 5 Jan, and week 52 ends on 28 Dec. 1 January 2025 sits in week 1 of 2025 and 31 December 2025 in week 1 of 2026.
| Week | Monday | Sunday | Month | Falls in this week |
|---|---|---|---|---|
| 1 | 30 Dec 2024 | 5 Jan | January | New Year's Day, New Year's Eve |
| 2 | 6 Jan | 12 Jan | January | |
| 3 | 13 Jan | 19 Jan | January | |
| 4 | 20 Jan | 26 Jan | January | |
| 5 | 27 Jan | 2 Feb | January | Groundhog Day |
| 6 | 3 Feb | 9 Feb | February | |
| 7 | 10 Feb | 16 Feb | February | Valentine's Day |
| 8 | 17 Feb | 23 Feb | February | |
| 9 | 24 Feb | 2 Mar | February | Spring |
| 10 | 3 Mar | 9 Mar | March | Mardi Gras, International Women's Day |
| 11 | 10 Mar | 16 Mar | March | Pi Day |
| 12 | 17 Mar | 23 Mar | March | St. Patrick's Day, the Spring Equinox, World Water Day, the International Day of Happiness |
| 13 | 24 Mar | 30 Mar | March | Earth Hour |
| 14 | 31 Mar | 6 Apr | April | April Fools' Day |
| 15 | 7 Apr | 13 Apr | April | World Health Day, National Pet Day |
| 16 | 14 Apr | 20 Apr | April | Easter, Good Friday, Tax Day |
| 17 | 21 Apr | 27 Apr | April | Earth Day, World Book Day |
| 18 | 28 Apr | 4 May | May | Star Wars Day, May Day |
| 19 | 5 May | 11 May | May | Mother's Day, Cinco de Mayo |
| 20 | 12 May | 18 May | May | |
| 21 | 19 May | 25 May | May | |
| 22 | 26 May | 1 Jun | May | Summer, Memorial Day |
| 23 | 2 Jun | 8 Jun | June | World Environment Day, World Oceans Day |
| 24 | 9 Jun | 15 Jun | June | Father's Day |
| 25 | 16 Jun | 22 Jun | June | Juneteenth, the Summer Solstice |
| 26 | 23 Jun | 29 Jun | June | |
| 27 | 30 Jun | 6 Jul | July | Independence Day |
| 28 | 7 Jul | 13 Jul | July | |
| 29 | 14 Jul | 20 Jul | July | World Emoji Day |
| 30 | 21 Jul | 27 Jul | July | |
| 31 | 28 Jul | 3 Aug | July | |
| 32 | 4 Aug | 10 Aug | August | |
| 33 | 11 Aug | 17 Aug | August | Back to School |
| 34 | 18 Aug | 24 Aug | August | World Photography Day |
| 35 | 25 Aug | 31 Aug | August | National Dog Day |
| 36 | 1 Sep | 7 Sep | September | Labor Day, Autumn |
| 37 | 8 Sep | 14 Sep | September | |
| 38 | 15 Sep | 21 Sep | September | the International Day of Peace, Talk Like a Pirate Day |
| 39 | 22 Sep | 28 Sep | September | the Autumn Equinox |
| 40 | 29 Sep | 5 Oct | October | World Teachers' Day, International Coffee Day, National Coffee Day |
| 41 | 6 Oct | 12 Oct | October | World Mental Health Day |
| 42 | 13 Oct | 19 Oct | October | Columbus Day, World Food Day |
| 43 | 20 Oct | 26 Oct | October | Diwali |
| 44 | 27 Oct | 2 Nov | October | Halloween, National Cat Day |
| 45 | 3 Nov | 9 Nov | November | |
| 46 | 10 Nov | 16 Nov | November | Veterans Day, Singles' Day, World Kindness Day |
| 47 | 17 Nov | 23 Nov | November | International Men's Day, World Television Day |
| 48 | 24 Nov | 30 Nov | November | Thanksgiving, Black Friday |
| 49 | 1 Dec | 7 Dec | December | Cyber Monday, Winter, Giving Tuesday |
| 50 | 8 Dec | 14 Dec | December | Human Rights Day |
| 51 | 15 Dec | 21 Dec | December | the Winter Solstice |
| 52 | 22 Dec | 28 Dec | December | Christmas, Boxing Day |
Why 2025 has 52 weeks
An ISO week year has 53 weeks when 1 January falls on a Thursday, or when the year is a leap year whose 1 January falls on a Wednesday. Every other year has 52. Because 52 weeks is only 364 days, the calendar has to absorb the leftover day or two somewhere, and a 53-week year is where it does that. Payroll systems, retail reporting and production planning all trip over this, because a 53-week year has one extra reporting period.
The year boundary
The first and last weeks of an ISO year rarely line up with the calendar year. Week 1 of 2025 starts on 30 Dec 2024, and 1 January 2025 belongs to week 1 of 2025. This is not an error in the calendar: it follows directly from the rule that week 1 is the week containing the first Thursday, which keeps every week whole and every week in exactly one year.
Other years
The same calendar for 2020 to 2035. Years with 53 weeks are marked.
Frequently asked questions
How many weeks are in 2025?
2025 has 52 ISO 8601 weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday; otherwise it has 52.
Which week is 1 January 2025 in?
1 January 2025 falls in ISO week 1 of 2025.
Which week is 31 December 2025 in?
31 December 2025 falls in ISO week 1 of 2026. That week already belongs to 2026, because most of it sits in the next year.
When does an ISO week start?
On Monday. ISO 8601 defines the week as running Monday to Sunday, and week 1 as the week containing the first Thursday of the year, which is the same as the week containing 4 January. That is why an ISO week calendar can open in late December.
Need the week number for one specific date instead? Use the week number tool. Want this calendar as data? It is published as free JSON at /api/weeks/2025.json, documented on the data page.
Computed from the ISO 8601 rule, not from a stored table. Last updated 2026-08-05.
Related tools
Keep the date math going with the rest of the toolkit.