Day of Week Calculator
Find the day of the week, week number, and day of year for any date.
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How to use this calculator
- 1
Select any date using the date picker.
- 2
Read the weekday name and the long-form date to confirm the right day was picked.
- 3
Check the week line: it names the ISO week and the ISO year, which can differ from the calendar year near January 1.
- 4
Use the day-of-year and days-left lines for ordinal dates, then the weekend line for the gap to Saturday.
Frequently asked questions
Which week-numbering system does it use?
ISO 8601: weeks run Monday to Sunday and week 1 is the week containing the first Thursday of the year, which is the same as the week containing January 4. January 1, 2026 is a Thursday, so it sits in week 1 of 2026.
Why does December 29, 2025 show “Week 1 of 2026”?
Under ISO rules a week belongs to the year that holds its Thursday. That Monday is in the week ending Sunday January 4, 2026, so the week is week 1 of 2026 even though the date is in 2025. The tool shows this ISO year, not the calendar year, next to the week number.
Can a year have 53 weeks?
Yes. 2026 does: December 31, 2026 and January 1, 2027 both report Week 53 of 2026. Many years have 52, and the extra week appears when the year starts on a Thursday, or on a Wednesday in a leap year.
How does it decide a year is a leap year?
A year is a leap year if it divides by 4, except century years, which must divide by 400. So 2024 has 366 days (February 29, 2024 is day 60 of 366), 1900 does not, and 2000 does. January 1, 2000 shows as day 1 of 366.
What does “days left in year” count?
The days after the selected date, so the date itself is not included. December 31 shows 0 and November 3, 2026, which is day 307, shows 58.
What does the weekend line tell me?
On Monday to Friday it shows how many days until Saturday: 4 days from Tuesday, 1 day from Friday. On Saturday or Sunday it says it is a weekend. It does not account for countries with a different weekend.
Is it reliable for historical dates?
It uses the Gregorian calendar for all dates, including those before it was adopted, so a date in 1500 shows the weekday the modern calendar would assign, not what contemporaries used. Enter four-digit years; very early years below 100 can be misread as 19xx.
Day of week calculator — weekday, ISO week number and day of year
Three ways to place a date in the calendar
The weekday tells you where a date sits in the week; the ISO week number is the planning label used in schedules and shipping documents; the day of the year is a running count from January 1 used in logs and ordinal dates. The calculator gives all three, plus days left in the year and the distance to the weekend. Enter any date from the past or future, such as a birth date, an anniversary or a planned launch, and the answer is the same regardless of which device you use.
Worked examples
Each date was run through the calculator on its own, with no reference to the current day. Two further rows cover a leap year that ends on its 366th day and a Sunday that still belongs to the previous ISO year.
| Date | Weekday | ISO week | Day of year | Days left |
|---|---|---|---|---|
| 2026-11-03 | Tuesday | Week 45 of 2026 | Day 307 of 365 | 58 |
| 2024-02-29 | Thursday | Week 9 of 2024 | Day 60 of 366 | 306 |
| 2025-12-29 | Monday | Week 1 of 2026 | Day 363 of 365 | 2 |
| 2027-01-01 | Friday | Week 53 of 2026 | Day 1 of 365 | 364 |
| 2000-01-01 | Saturday | Week 52 of 1999 | Day 1 of 366 | 365 |
| 2020-12-31 | Thursday | Week 53 of 2020 | Day 366 of 366 | 0 |
| 2021-01-03 | Sunday | Week 53 of 2020 | Day 3 of 365 | 362 |
| 1900-03-01 | Thursday | Week 9 of 1900 | Day 60 of 365 | 305 |
When the week year and calendar year disagree
The three middle rows show the quirk. December 29, 2025 is in calendar year 2025 but in ISO week 1 of 2026, because that week’s Thursday is January 1. January 1, 2027 is in calendar 2027 but belongs to week 53 of 2026. January 1, 2000 falls in week 52 of 1999. Anyone building reports by week should group by the ISO year, not by the date’s own year.
Leap years and the day count
Day of year depends on whether February has 29 days. February 29, 2024 is day 60 of 366, and a leap year makes every later date one day higher than in a common year. Remember the rule: divisible by 4, except centuries, which must also divide by 400. The check works at the other end of the table too: March 1, 1900 is day 60 of 365 because 1900 is a century year that is not divisible by 400, so it had no February 29, while March 1, 2000 would be day 61. December 31, 2020 closes a leap year at day 366 with 0 days left.
Planning around the weekend
The final line counts the days until Saturday from any weekday, and says it is a weekend on Saturday or Sunday; November 7 and 8, 2026 both read that way. An emptied date field returns a dash. A Tuesday shows 4 days, a Friday 1 day. It assumes a Saturday–Sunday weekend and does not know about holidays. Dates are interpreted in the Gregorian calendar throughout, including earlier ones. This keeps results consistent, and it means that a very old date can differ from the weekday recorded in a contemporary document.
Learn more from an authoritative source:
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →