The 2027 Calendar Year has 52 weeks under the ISO 8601 standard. Week 1 of 2027 runs from Monday, January 4, to Sunday, January 10, 2027, and week 52 ends on Sunday, January 2, 2028.
The US system numbers weeks differently, so this page shows both, with every 2027 week, its dates, and the holidays it holds.
Key takeaways
- 2027 has 52 ISO weeks; week 1 starts on Monday, January 4, 2027.
- January 1–3, 2027, belong to week 53 of 2026, because 2026 is a 53-week year.
- In the US system, January 1, 2027, is week 1, and the year ends in week 53.
- In 2027, the US week number is 1 higher than the ISO number from Monday to Saturday, and 2 higher on Sundays.
Week numbers 2027 at a glance
- Number of ISO weeks in 2027: 52.
- ISO week 1: Monday, January 4 – Sunday, January 10, 2027.
- Last ISO week (52): Monday, December 27, 2027 – Sunday, January 2, 2028.
- January 1, 2027: a Friday, in ISO week 53 of 2026 and US week 1 of 2027.
- Number of US weeks in 2027: 53, with week 1 only 2 days long (January 1–2).
- ISO weeks start on: Monday. US weeks start on: Sunday.
Table of Contents
2027 week numbers list with dates
The table lists all 52 ISO weeks of 2027, Monday to Sunday, with the US week number that covers the same Monday to Saturday.
Holidays are US federal holidays, plus Easter, daylight saving time, and other widely marked days.
| ISO week | Dates (Monday – Sunday) | US week (Monday – Saturday) | Holidays and key dates |
|---|---|---|---|
| 53 of 2026 | December 28, 2026 – January 3, 2027 | 1 (January 1–2) | New Year’s Day (Friday, January 1) |
| 1 | January 4–10 | 2 | |
| 2 | January 11–17 | 3 | |
| 3 | January 18–24 | 4 | Martin Luther King Jr. Day (Monday, January 18) |
| 4 | January 25–31 | 5 | |
| 5 | February 1–7 | 6 | |
| 6 | February 8–14 | 7 | Valentine’s Day (Sunday, February 14) |
| 7 | February 15–21 | 8 | Presidents’ Day (Monday, February 15) |
| 8 | February 22–28 | 9 | |
| 9 | March 1–7 | 10 | |
| 10 | March 8–14 | 11 | Daylight saving time starts (Sunday, March 14) |
| 11 | March 15–21 | 12 | |
| 12 | March 22–28 | 13 | Good Friday (March 26); Easter Sunday (March 28) |
| 13 | March 29 – April 4 | 14 | |
| 14 | April 5–11 | 15 | |
| 15 | April 12–18 | 16 | |
| 16 | April 19–25 | 17 | |
| 17 | April 26 – May 2 | 18 | |
| 18 | May 3–9 | 19 | Mother’s Day (Sunday, May 9) |
| 19 | May 10–16 | 20 | |
| 20 | May 17–23 | 21 | |
| 21 | May 24–30 | 22 | |
| 22 | May 31 – June 6 | 23 | Memorial Day (Monday, May 31) |
| 23 | June 7–13 | 24 | |
| 24 | June 14–20 | 25 | Juneteenth, observed (Friday, June 18); Father’s Day (Sunday, June 20) |
| 25 | June 21–27 | 26 | |
| 26 | June 28 – July 4 | 27 | Independence Day (Sunday, July 4) |
| 27 | July 5–11 | 28 | Independence Day, observed (Monday, July 5) |
| 28 | July 12–18 | 29 | |
| 29 | July 19–25 | 30 | |
| 30 | July 26 – August 1 | 31 | |
| 31 | August 2–8 | 32 | |
| 32 | August 9–15 | 33 | |
| 33 | August 16–22 | 34 | |
| 34 | August 23–29 | 35 | |
| 35 | August 30 – September 5 | 36 | |
| 36 | September 6–12 | 37 | Labor Day (Monday, September 6) |
| 37 | September 13–19 | 38 | |
| 38 | September 20–26 | 39 | |
| 39 | September 27 – October 3 | 40 | |
| 40 | October 4–10 | 41 | |
| 41 | October 11–17 | 42 | Columbus Day and Indigenous Peoples’ Day (Monday, October 11) |
| 42 | October 18–24 | 43 | |
| 43 | October 25–31 | 44 | Halloween (Sunday, October 31) |
| 44 | November 1–7 | 45 | Daylight saving time ends (Sunday, November 7) |
| 45 | November 8–14 | 46 | Veterans Day (Thursday, November 11) |
| 46 | November 15–21 | 47 | |
| 47 | November 22–28 | 48 | Thanksgiving (Thursday, November 25); Black Friday (November 26) |
| 48 | November 29 – December 5 | 49 | |
| 49 | December 6–12 | 50 | |
| 50 | December 13–19 | 51 | |
| 51 | December 20–26 | 52 | Christmas Day, observed (Friday, December 24); Christmas Day (Saturday, December 25) |
| 52 | December 27 – January 2, 2028 | 53 | New Year’s Day 2028, observed (Friday, December 31) |
Each US week starts one day before the ISO week, on Sunday. For example, US week 2 runs from Sunday, January 3, to Saturday, January 9, 2027. All 2027 holidays and observances are also marked month by month in the 2027 calendar.
ISO week numbers vs US week numbers
The ISO and US systems give different week numbers for the same date because they start the year and the week differently. ISO 8601 weeks start on Monday, and week 1 is the week with the year’s first Thursday. US weeks start on Sunday, and week 1 is the week that contains January 1.
| Rule | ISO 8601 system | US system |
|---|---|---|
| First day of the week | Monday | Sunday |
| Week 1 | The week containing the first Thursday (always contains January 4) | The week containing January 1 |
| Week 1 of 2027 | January 4–10, 2027 | January 1–2, 2027 |
| Weeks in 2027 | 52, all full weeks | 53, with partial weeks at both ends |
| Can early January belong to the year before? | Yes: January 1–3, 2027, are in week 53 of 2026 | No: January 1 is always week 1 |
| Excel formula | =ISOWEEKNUM(date) or =WEEKNUM(date,21) | =WEEKNUM(date) or =WEEKNUM(date,1) |
| Main users | Europe and international business | United States, Canada, and many US-based software tools |
Microsoft documents both rules in its WEEKNUM function guide. It calls the US rule “System 1,” where “the week containing January 1 is the first week of the year,” and the ISO rule “System 2,” where “the week containing the first Thursday of the year is the first week of the year.”
By our count, the two systems never agree on a single day in 2027. The US number is 1 higher than the ISO number on 311 days and 2 higher on the 51 Sundays from January 10 to December 26. On January 1–3, the ISO system is still in 2026.
Which one should you use? Use ISO week numbers when you work with European or international teams, suppliers, or schools, because ISO is the standard there.
Use US week numbers for US payroll, US retail, or a spreadsheet already set to WEEKNUM’s default. If a document just says “week 12,” check which system it uses before you plan around it.
How many weeks are in 2027?
2027 has 52 ISO weeks. Any year has 52 weeks plus 1 extra day, or 2 extra days in a leap year, and the ISO system collects those extra days into a 53rd week every 5 to 7 years.
An ISO year has 53 weeks when January 1 falls on a Thursday, or when a leap year starts on a Wednesday (ISO week date). January 1, 2026, is a Thursday, so 2026 has 53 weeks. January 1, 2027, is a Friday, so 2027 has 52.
Work weeks and fiscal weeks in 2027
A work week number is usually the same as the calendar week number, but each company picks the system it uses. Many US companies use the US system, while European and global firms mostly use ISO weeks.
Fiscal week numbers are different again. A fiscal week counts from the start of the company’s fiscal year, not from January 1. Retailers often use a 52- or 53-week fiscal year split into 4-5-4 quarters, so fiscal week 1 may fall in February. Check your company’s payroll or finance calendar for its fiscal week dates.
A 52-week ISO year divides into four quarters of 13 weeks each. In 2027, ISO weeks 1–13 run from January 4 to April 4, weeks 14–26 to July 4, weeks 27–39 to October 3, and weeks 40–52 to January 2, 2028.
How to read an ISO week date
An ISO week date writes a day as the year, the letter W, the week number, and the day number. Days are numbered 1 for Monday to 7 for Sunday.
- 2027-W01-1 is Monday, January 4, 2027.
- 2027-W12-7 is Sunday, March 28, 2027 (Easter Sunday).
- 2026-W53-5 is Friday, January 1, 2027.
- 2027-W52 is the week of December 27, 2027, to January 2, 2028.
The standard’s short form drops the hyphens, so 2027-W01-1 can also appear as 2027W011.
The year in a week date is the ISO week year, which is why January 1, 2027, is written with 2026.
Week numbers 2026 to 2032
ISO week 1 starts between December 29 and January 4. The table shows when week 1 starts each year and how many ISO weeks the year has.
| Year | January 1 falls on | ISO week 1 starts | ISO weeks in the year |
|---|---|---|---|
| 2026 | Thursday | Monday, December 29, 2025 | 53 |
| 2027 | Friday | Monday, January 4, 2027 | 52 |
| 2028 | Saturday | Monday, January 3, 2028 | 52 |
| 2029 | Monday | Monday, January 1, 2029 | 52 |
| 2030 | Tuesday | Monday, December 31, 2029 | 52 |
| 2031 | Wednesday | Monday, December 30, 2030 | 52 |
| 2032 | Thursday | Monday, December 29, 2031 | 53 |
After 2026, the next 53-week years are 2032, 2037, and 2043. Over the 400-year cycle of the Gregorian calendar, 71 years have 53 ISO weeks, and 329 have 52.
Frequently asked questions
What week number is January 1, 2027?
January 1, 2027, is in ISO week 53 of 2026 and in US week 1 of 2027. ISO week 1 of 2027 starts on Monday, January 4.
Does 2027 have 52 or 53 weeks?
2027 has 52 ISO weeks. In the US system, which counts partial weeks, 2027 runs to week 53.
What week is Easter 2027?
Easter Sunday, March 28, 2027, is in ISO week 12. In the US system, it starts week 14.
What week is Thanksgiving 2027?
Thanksgiving, Thursday, November 25, 2027, is in ISO week 47 and US week 48.
When does week 52 of 2027 start?
ISO week 52 of 2027 starts on Monday, December 27, 2027, and ends on Sunday, January 2, 2028.
Why does my calendar show a different week number?
Your calendar most likely uses the US system, while this page leads with ISO weeks. In 2027, the US number is 1 or 2 higher than the ISO number for the same date.
How do I get the week number in Excel?
Use =ISOWEEKNUM(A1) for the ISO week number and =WEEKNUM(A1) for the US week number, where A1 holds the date.
