📅 Day of the Week Calculator
By ToolNimba Editorial Team · Updated 2026-06-19
Pick a date to see the weekday, the day of the year, and its ISO week number.
This day of the week calculator tells you which weekday any date falls on, from your birthday to a deadline years in the future. Pick a date and you will instantly see the weekday name (Monday through Sunday), the day of the year (a number from 1 to 365 or 366), and the ISO 8601 week number. It works for past and future dates alike, so you can answer questions like "what day was I born?" or "which weekday is New Year next year?" without flipping through a calendar.
What is the Day of the Week Calculator?
Every date on the Gregorian calendar maps to exactly one of seven weekdays, and that mapping follows a fixed pattern. A common year of 365 days is 52 weeks plus one extra day, so the weekday of a given date normally shifts forward by one each year. A leap year adds a second extra day, which is why dates after the end of February can jump forward by two weekdays across a leap year. The calculator does this for you by reading the actual calendar, so you never have to track the shift by hand.
The day of the year is simply the count of days from January 1 (which is day 1) up to and including the date you chose. The maximum is 365 in a common year and 366 in a leap year, because of the extra day on February 29. This number is handy for planning, logistics and scientific work, where it is sometimes called the ordinal date. For example, the 100th day of the year is usually April 10, or April 9 in a leap year.
The ISO week number follows the ISO 8601 standard used across most of the world for business and software. Weeks start on Monday, and week 1 is defined as the week containing the first Thursday of the year (equivalently, the week containing January 4). A quirk of this rule is that the first few days of January can belong to the last week (52 or 53) of the previous year, and the last days of December can belong to week 1 of the next year. The calculator shows the ISO week-year in brackets whenever it differs from the calendar year, so the result is never ambiguous.
When to use it
- Finding out what day of the week you, a child or a famous person was born on.
- Checking which weekday a future date such as a wedding, deadline or holiday falls on.
- Working out the day of the year (ordinal date) for planning, logistics or scientific records.
- Looking up the ISO week number used in payroll, project schedules and spreadsheet reporting.
How to use the Day of the Week Calculator
- Enter or pick a date using the date field (it starts on today by default).
- Read off the weekday name in the first result box.
- Check the day of the year and the total days for that year in the second box.
- See the ISO 8601 week number in the third box, with the ISO week-year shown when it differs from the calendar year.
Formula & method
Worked examples
What day of the week was 20 July 1969, the first Moon landing?
- Read the weekday from the calendar: 20 July 1969 is a Sunday.
- Day of the year: 31 (Jan) + 28 (Feb) + 31 (Mar) + 30 (Apr) + 31 (May) + 30 (Jun) = 181 days through June.
- Add the 20 days of July: 181 + 20 = 201.
- 1969 is not a leap year, so the year has 365 days.
- ISO week: 20 July 1969 falls in ISO week 29.
Result: Sunday, day 201 of 365, ISO week 29
What day of the week was 1 January 2000, and which ISO week is it?
- Read the weekday from the calendar: 1 January 2000 is a Saturday.
- Day of the year: 1 January is always day 1.
- 2000 is a leap year (divisible by 400), so the year has 366 days.
- ISO week 1 of 2000 starts on Monday 3 January, the week containing the first Thursday (6 January).
- 1 January 2000 is a Saturday before that Monday, so it belongs to the last ISO week of 1999, which is week 52.
Result: Saturday, day 1 of 366, ISO week 52 (1999)
How the weekday of a fixed date shifts from year to year (example: 1 March)
| Date | Weekday | Note |
|---|---|---|
| 1 March 2024 | Friday | 2024 is a leap year |
| 1 March 2025 | Saturday | Shifted forward 2 days across the leap year |
| 1 March 2026 | Sunday | Common year, shift of 1 day |
| 1 March 2027 | Monday | Common year, shift of 1 day |
Day of the year for the 1st of each month (common year vs leap year)
| Month (1st) | Common year | Leap year |
|---|---|---|
| January | 1 | 1 |
| March | 60 | 61 |
| July | 182 | 183 |
| October | 274 | 275 |
| December | 335 | 336 |
Common mistakes to avoid
- Confusing the ISO week number with a simple calendar-week count. The ISO 8601 week is not just the date divided by seven. Weeks run Monday to Sunday and week 1 is the one containing the first Thursday of the year, so early January days can fall in week 52 or 53 of the previous year.
- Forgetting that leap years shift weekdays by two. A date after February in a leap year moves forward two weekdays compared with the previous year, not one. That is why a fixed date like 1 March can skip a weekday across a leap year.
- Mixing up time zones. A weekday depends on the local date. This tool reads the date at local midnight, so the answer is for the calendar date you typed, not a shifted UTC date.
- Assuming the day of the year always tops out at 365. In a leap year there are 366 days because of 29 February, so any date from 1 March onward has a day-of-year number one higher than in a common year.
Glossary
- Weekday
- One of the seven named days from Monday to Sunday that a calendar date falls on.
- Day of the year
- The position of a date counted from 1 January (day 1) to 31 December (day 365 or 366). Also called the ordinal date.
- ISO week number
- A week index from 1 to 53 defined by ISO 8601, where week 1 contains the first Thursday of the year and weeks start on Monday.
- Leap year
- A year with 366 days, occurring when the year is divisible by 4 but not 100, or divisible by 400. It adds 29 February.
- Gregorian calendar
- The standard civil calendar used in most of the world today, introduced in 1582.
Frequently asked questions
What day of the week was I born on?
Enter your date of birth in the date field and the calculator shows the weekday straight away, along with the day of the year and ISO week number. It works for any date in the past, including decades ago.
How does the calculator find the weekday?
It reads your chosen date directly from the Gregorian calendar at local midnight, then returns the matching weekday name. There is no rounding or guesswork, so the result is exact for any valid date.
What is the day of the year?
The day of the year is the count of days from 1 January, which is day 1, up to and including your date. It runs to 365 in a common year and 366 in a leap year because of 29 February. It is sometimes called the ordinal date.
What is the ISO week number?
It is the week index defined by the ISO 8601 standard. Weeks start on Monday and week 1 is the week that contains the first Thursday of the year. This is the week numbering used in payroll, project planning and many spreadsheets.
Why does 1 January sometimes show a week number with a different year?
Under ISO 8601 the first days of January can belong to the final week of the previous year. When that happens the calculator shows the ISO week-year in brackets, for example Week 52 (1999), so the result stays unambiguous.
Does it work for future dates?
Yes. The Gregorian calendar pattern is fixed, so the calculator gives the correct weekday, day of the year and ISO week for any future date as well as past ones, with no network connection needed.