How many weeks are in 2027?
2027 has 52 weeks in ISO 8601, because it starts on a Friday. In the US system, where week 1 is the week of January 1 and partial weeks count, 2027 runs from week 1 to week 53. The last US week starts on Sunday, Dec 26, 2027.
January 1 to 3, 2027 still belong to ISO week 53 of 2026, so ISO week 1 starts on Monday, Jan 4, 2027. The last ISO week, week 52, ends on Sunday, Jan 2, 2028.
US week 1 runs from Dec 27, 2026 to Jan 2, 2027. Because US weeks start on Sunday and ISO weeks on Monday, the same number never covers exactly the same days. In 2027 the gap is a full week for most of the year: from Monday to Saturday, the US number is one higher than the ISO number (ISO week 10 is US week 11), and on Sundays it is two higher.
Weeks month by month
January: US weeks 1 to 6; February: US weeks 6 to 10; March: US weeks 10 to 14; April: US weeks 14 to 18; May: US weeks 18 to 23; June: US weeks 23 to 27; July: US weeks 27 to 31; August: US weeks 32 to 36; September: US weeks 36 to 40; October: US weeks 40 to 45; November: US weeks 45 to 49; December: US weeks 49 to 53. In ISO numbering, January 2027 starts in week 53 (of 2026) and December ends in week 52.
2027 week numbers with dates
| Week | US (Sunday to Saturday) | ISO 8601 (Monday to Sunday) |
|---|---|---|
| 1 | Dec 27, 2026 to Jan 2 | Jan 4 to Jan 10 |
| 2 | Jan 3 to Jan 9 | Jan 11 to Jan 17 |
| 3 | Jan 10 to Jan 16 | Jan 18 to Jan 24 |
| 4 | Jan 17 to Jan 23 | Jan 25 to Jan 31 |
| 5 | Jan 24 to Jan 30 | Feb 1 to Feb 7 |
| 6 | Jan 31 to Feb 6 | Feb 8 to Feb 14 |
| 7 | Feb 7 to Feb 13 | Feb 15 to Feb 21 |
| 8 | Feb 14 to Feb 20 | Feb 22 to Feb 28 |
| 9 | Feb 21 to Feb 27 | Mar 1 to Mar 7 |
| 10 | Feb 28 to Mar 6 | Mar 8 to Mar 14 |
| 11 | Mar 7 to Mar 13 | Mar 15 to Mar 21 |
| 12 | Mar 14 to Mar 20 | Mar 22 to Mar 28 |
| 13 | Mar 21 to Mar 27 | Mar 29 to Apr 4 |
| 14 | Mar 28 to Apr 3 | Apr 5 to Apr 11 |
| 15 | Apr 4 to Apr 10 | Apr 12 to Apr 18 |
| 16 | Apr 11 to Apr 17 | Apr 19 to Apr 25 |
| 17 | Apr 18 to Apr 24 | Apr 26 to May 2 |
| 18 | Apr 25 to May 1 | May 3 to May 9 |
| 19 | May 2 to May 8 | May 10 to May 16 |
| 20 | May 9 to May 15 | May 17 to May 23 |
| 21 | May 16 to May 22 | May 24 to May 30 |
| 22 | May 23 to May 29 | May 31 to Jun 6 |
| 23 | May 30 to Jun 5 | Jun 7 to Jun 13 |
| 24 | Jun 6 to Jun 12 | Jun 14 to Jun 20 |
| 25 | Jun 13 to Jun 19 | Jun 21 to Jun 27 |
| 26 | Jun 20 to Jun 26 | Jun 28 to Jul 4 |
| 27 | Jun 27 to Jul 3 | Jul 5 to Jul 11 |
| 28 | Jul 4 to Jul 10 | Jul 12 to Jul 18 |
| 29 | Jul 11 to Jul 17 | Jul 19 to Jul 25 |
| 30 | Jul 18 to Jul 24 | Jul 26 to Aug 1 |
| 31 | Jul 25 to Jul 31 | Aug 2 to Aug 8 |
| 32 | Aug 1 to Aug 7 | Aug 9 to Aug 15 |
| 33 | Aug 8 to Aug 14 | Aug 16 to Aug 22 |
| 34 | Aug 15 to Aug 21 | Aug 23 to Aug 29 |
| 35 | Aug 22 to Aug 28 | Aug 30 to Sep 5 |
| 36 | Aug 29 to Sep 4 | Sep 6 to Sep 12 |
| 37 | Sep 5 to Sep 11 | Sep 13 to Sep 19 |
| 38 | Sep 12 to Sep 18 | Sep 20 to Sep 26 |
| 39 | Sep 19 to Sep 25 | Sep 27 to Oct 3 |
| 40 | Sep 26 to Oct 2 | Oct 4 to Oct 10 |
| 41 | Oct 3 to Oct 9 | Oct 11 to Oct 17 |
| 42 | Oct 10 to Oct 16 | Oct 18 to Oct 24 |
| 43 | Oct 17 to Oct 23 | Oct 25 to Oct 31 |
| 44 | Oct 24 to Oct 30 | Nov 1 to Nov 7 |
| 45 | Oct 31 to Nov 6 | Nov 8 to Nov 14 |
| 46 | Nov 7 to Nov 13 | Nov 15 to Nov 21 |
| 47 | Nov 14 to Nov 20 | Nov 22 to Nov 28 |
| 48 | Nov 21 to Nov 27 | Nov 29 to Dec 5 |
| 49 | Nov 28 to Dec 4 | Dec 6 to Dec 12 |
| 50 | Dec 5 to Dec 11 | Dec 13 to Dec 19 |
| 51 | Dec 12 to Dec 18 | Dec 20 to Dec 26 |
| 52 | Dec 19 to Dec 25 | Dec 27 to Jan 2, 2028 |
| 53 | Dec 26 to Jan 1, 2028 | No ISO week 53 |
Holidays by week number in 2027
Federal holidays and Easter, with their week in each system. 12 of these 12 dates have a different number in the two systems, so say which one you mean when you plan around them. Falling on a weekend this year: Juneteenth (Saturday, observed Fri, Jun 18); Independence Day (Sunday, observed Mon, Jul 5); Christmas Day (Saturday, observed Fri, Dec 24).
| Holiday | Date | US week | ISO week |
|---|---|---|---|
| New Year’s Day | Fri, Jan 1 | 1 | 53 |
| Martin Luther King Jr. Day | Mon, Jan 18 | 4 | 3 |
| Presidents’ Day | Mon, Feb 15 | 8 | 7 |
| Easter Sunday | Sun, Mar 28 | 14 | 12 |
| Memorial Day | Mon, May 31 | 23 | 22 |
| Juneteenth | Sat, Jun 19 (observed Fri, Jun 18) | 25 | 24 |
| Independence Day | Sun, Jul 4 (observed Mon, Jul 5) | 28 | 26 |
| Labor Day | Mon, Sep 6 | 37 | 36 |
| Columbus Day | Mon, Oct 11 | 42 | 41 |
| Veterans Day | Thu, Nov 11 | 46 | 45 |
| Thanksgiving | Thu, Nov 25 | 48 | 47 |
| Christmas Day | Sat, Dec 25 (observed Fri, Dec 24) | 52 | 51 |
Writing a date as an ISO week date
ISO 8601 can also write a date as year, week and weekday. Thanksgiving 2027, Nov 25, 2027, becomes 2027-W47-4: week 47, day 4 (Thursday). Christmas Day becomes 2027-W51-6. Planning tools and some spreadsheets accept this form directly.
Other years: week numbers 2026 and week numbers 2028. For any single date, the week number tool gives the week in one tap, and the day of the week calculator tells you which weekday it falls on.