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Most of Europe and many businesses use ISO 8601 week numbers: weeks start on Monday and week 1 is the week with the year's first Thursday. Some years have 53 weeks, as 2026 does.
In the US, week numbers often start on Sunday with 1 January always in week 1, so the two systems can differ by one.
| Date (blank = today) | 14 Nov 2026 |
| ISO week number | Week 46 of 2026 |
| Week runs | Mon 9 Nov 2026 – Sun 15 Nov 2026 |
| US week number (Sunday start) | Week 46 |
| Day of the year | 318 of 365 |
| ISO weeks in 2026 | 53 |
| Quarter | Q4 |
Week 46 of 2026.
| Date (blank = today) | 1 Jan 2027 |
| ISO week number | Week 53 of 2026 |
| Week runs | Mon 28 Dec 2026 – Sun 3 Jan 2027 |
| US week number (Sunday start) | Week 1 |
| Day of the year | 1 of 365 |
| ISO weeks in 2027 | 52 |
| Quarter | Q1 |
Week 53 of 2026.
53 ISO weeks, because 2026 starts on a Thursday.
Friday 25 December 2026 is in ISO week 52.
Under ISO rules, if 1 January falls on Friday to Sunday it belongs to the last week of the previous year.
ISO weeks start on Monday; US calendars usually start on Sunday.