Week Numbers 2021
2021 has 52 numbered weeks under ISO 8601 and 53 under the US convention. Week 1 starts on Monday 4 January 2021 and week 52 ends on Sunday 2 January 2022.
Today in UTC.
Weeks run Monday to Sunday. Week 1 holds the first Thursday.
Week number now
of 52 in 2021 · ISO 8601 numbering
Monday 4 January 2021 is in week 1 of 2021 under ISO 8601 numbering. That week runs from Monday 4 January 2021 to Sunday 10 January 2021.
The two conventions disagree on this date. US numbering makes it week 2 of 2021. Check which one the system you are filling in uses, because a week number without its convention is not a complete answer.
Show the working
| Date | 2021-01-04 |
|---|---|
| Weekday | Monday |
| ISO week 1 starts | 2021-01-04 |
| US week 1 starts | 2020-12-27 |
| This week runs | 2021-01-04 to 2021-01-10 |
| Weeks in 2021 | 52 weeks |
Every week of 2021
ISO 8601 weeks, running Monday to Sunday. Week 1 is the week holding the first Thursday of the year, which is why it can start in the previous December.
| Week | Starts | Ends | ISO dates |
|---|---|---|---|
| Week 1 | 4 January 2021 | 10 January 2021 | 2021-01-04 to 2021-01-10 |
| Week 2 | 11 January 2021 | 17 January 2021 | 2021-01-11 to 2021-01-17 |
| Week 3 | 18 January 2021 | 24 January 2021 | 2021-01-18 to 2021-01-24 |
| Week 4 | 25 January 2021 | 31 January 2021 | 2021-01-25 to 2021-01-31 |
| Week 5 | 1 February 2021 | 7 February 2021 | 2021-02-01 to 2021-02-07 |
| Week 6 | 8 February 2021 | 14 February 2021 | 2021-02-08 to 2021-02-14 |
| Week 7 | 15 February 2021 | 21 February 2021 | 2021-02-15 to 2021-02-21 |
| Week 8 | 22 February 2021 | 28 February 2021 | 2021-02-22 to 2021-02-28 |
| Week 9 | 1 March 2021 | 7 March 2021 | 2021-03-01 to 2021-03-07 |
| Week 10 | 8 March 2021 | 14 March 2021 | 2021-03-08 to 2021-03-14 |
| Week 11 | 15 March 2021 | 21 March 2021 | 2021-03-15 to 2021-03-21 |
| Week 12 | 22 March 2021 | 28 March 2021 | 2021-03-22 to 2021-03-28 |
| Week 13 | 29 March 2021 | 4 April 2021 | 2021-03-29 to 2021-04-04 |
| Week 14 | 5 April 2021 | 11 April 2021 | 2021-04-05 to 2021-04-11 |
| Week 15 | 12 April 2021 | 18 April 2021 | 2021-04-12 to 2021-04-18 |
| Week 16 | 19 April 2021 | 25 April 2021 | 2021-04-19 to 2021-04-25 |
| Week 17 | 26 April 2021 | 2 May 2021 | 2021-04-26 to 2021-05-02 |
| Week 18 | 3 May 2021 | 9 May 2021 | 2021-05-03 to 2021-05-09 |
| Week 19 | 10 May 2021 | 16 May 2021 | 2021-05-10 to 2021-05-16 |
| Week 20 | 17 May 2021 | 23 May 2021 | 2021-05-17 to 2021-05-23 |
| Week 21 | 24 May 2021 | 30 May 2021 | 2021-05-24 to 2021-05-30 |
| Week 22 | 31 May 2021 | 6 June 2021 | 2021-05-31 to 2021-06-06 |
| Week 23 | 7 June 2021 | 13 June 2021 | 2021-06-07 to 2021-06-13 |
| Week 24 | 14 June 2021 | 20 June 2021 | 2021-06-14 to 2021-06-20 |
| Week 25 | 21 June 2021 | 27 June 2021 | 2021-06-21 to 2021-06-27 |
| Week 26 | 28 June 2021 | 4 July 2021 | 2021-06-28 to 2021-07-04 |
| Week 27 | 5 July 2021 | 11 July 2021 | 2021-07-05 to 2021-07-11 |
| Week 28 | 12 July 2021 | 18 July 2021 | 2021-07-12 to 2021-07-18 |
| Week 29 | 19 July 2021 | 25 July 2021 | 2021-07-19 to 2021-07-25 |
| Week 30 | 26 July 2021 | 1 August 2021 | 2021-07-26 to 2021-08-01 |
| Week 31 | 2 August 2021 | 8 August 2021 | 2021-08-02 to 2021-08-08 |
| Week 32 | 9 August 2021 | 15 August 2021 | 2021-08-09 to 2021-08-15 |
| Week 33 | 16 August 2021 | 22 August 2021 | 2021-08-16 to 2021-08-22 |
| Week 34 | 23 August 2021 | 29 August 2021 | 2021-08-23 to 2021-08-29 |
| Week 35 | 30 August 2021 | 5 September 2021 | 2021-08-30 to 2021-09-05 |
| Week 36 | 6 September 2021 | 12 September 2021 | 2021-09-06 to 2021-09-12 |
| Week 37 | 13 September 2021 | 19 September 2021 | 2021-09-13 to 2021-09-19 |
| Week 38 | 20 September 2021 | 26 September 2021 | 2021-09-20 to 2021-09-26 |
| Week 39 | 27 September 2021 | 3 October 2021 | 2021-09-27 to 2021-10-03 |
| Week 40 | 4 October 2021 | 10 October 2021 | 2021-10-04 to 2021-10-10 |
| Week 41 | 11 October 2021 | 17 October 2021 | 2021-10-11 to 2021-10-17 |
| Week 42 | 18 October 2021 | 24 October 2021 | 2021-10-18 to 2021-10-24 |
| Week 43 | 25 October 2021 | 31 October 2021 | 2021-10-25 to 2021-10-31 |
| Week 44 | 1 November 2021 | 7 November 2021 | 2021-11-01 to 2021-11-07 |
| Week 45 | 8 November 2021 | 14 November 2021 | 2021-11-08 to 2021-11-14 |
| Week 46 | 15 November 2021 | 21 November 2021 | 2021-11-15 to 2021-11-21 |
| Week 47 | 22 November 2021 | 28 November 2021 | 2021-11-22 to 2021-11-28 |
| Week 48 | 29 November 2021 | 5 December 2021 | 2021-11-29 to 2021-12-05 |
| Week 49 | 6 December 2021 | 12 December 2021 | 2021-12-06 to 2021-12-12 |
| Week 50 | 13 December 2021 | 19 December 2021 | 2021-12-13 to 2021-12-19 |
| Week 51 | 20 December 2021 | 26 December 2021 | 2021-12-20 to 2021-12-26 |
| Week 52 | 27 December 2021 | 2 January 2022 | 2021-12-27 to 2022-01-02 |
2021 at a glance
| ISO 8601 weeks | 52 |
|---|---|
| US weeks | 53 |
| Days in the year | 365 |
| Leap year | No |
| 1 January falls on | Friday |
| 31 December falls on | Friday |
Why the first week can start in December
Week 1 of 2021 starts on Monday 4 January 2021. If that date is in December it is not a mistake. ISO 8601 assigns a whole week to the year holding most of its days, so a week straddling New Year belongs to one year or the other rather than being split in half.
The same rule at the other end means the last days of 2021 can fall in week 1 of 2022. Both are correct, and both are shown by the tool above when you enter a date near the boundary.
Frequently asked questions
How many weeks are in 2021?
52 under ISO 8601 numbering, and 53 under the US convention. 2021 is a common year of 365 days, which is 52 weeks and one day over.
When does week 1 of 2021 start?
Monday 4 January 2021 under ISO 8601, which runs Monday to Sunday. Under the US convention it starts on Sunday 27 December 2020. Both can begin in the previous December, which is normal.
Does 2021 have 53 weeks?
No. 2021 has 52 ISO weeks. A year only has 53 when it starts on a Thursday, or on a Wednesday in a leap year, and 1 January 2021 is a Friday.