📅 Day of the Week Calculator: Find What Day Any Date Falls On
By Shihab Mia · Updated 2026-07-07
Pick a date to see the weekday, the day of the year, and its ISO week number.
A day of the week calculator tells you exactly which weekday any calendar date falls on. Enter a date and you instantly see the weekday name (Monday through Sunday), the day of the year (a number from 1 to 365, or 366 in a leap year), and the ISO 8601 week number. It works for any date in the past or future, so you can answer "what day was I born?", "which weekday is my deadline?", or "what day is Christmas next year?" in one click, with no calendar flipping and no math. Every result is read straight from the Gregorian calendar, so it is exact for any valid date.
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, repeating pattern. A common year of 365 days is 52 weeks plus one extra day, so the weekday of a fixed date normally shifts forward by one each year. A leap year adds a second extra day (29 February), which is why dates from March onward can jump forward by two weekdays across a leap year. Because 365 and 366 share no clean multiple of 7, the full calendar pattern only repeats every 28 years (and exactly every 400 years once the century leap-year rule is included). The calculator sidesteps all of this by reading the actual calendar for the date you enter.
The day of the year is simply the count of days from 1 January (which is day 1) up to and including your chosen date. It tops out at 365 in a common year and 366 in a leap year. Scientists, logisticians and programmers call this the ordinal date, and it is handy for measuring how far through the year a date sits. As a quick reference, 1 July is day 182 in a common year and day 183 in a leap year, and the 100th day of the year is 10 April (9 April in a leap year).
The ISO week number follows the ISO 8601 standard used worldwide in business, payroll and software. Weeks start on Monday, and week 1 is defined as the week containing the first Thursday of the year, which is the same as the week containing 4 January. A consequence of this rule is that the first few days of January can belong to week 52 or 53 of the previous year, and the last days of December can fall in week 1 of the next year. This tool shows the ISO week-year in brackets whenever it differs from the calendar year, so a result like Week 52 (1999) is never ambiguous.
Behind the scenes, mathematicians have long had formulas to compute the weekday without a calendar. The best known is Zellers congruence, published by Christian Zeller in 1882, which uses modular arithmetic (everything reduced modulo 7) and treats January and February as months 13 and 14 of the previous year to handle leap years cleanly. A friendlier method for mental math is John Conways Doomsday rule from 1973, which anchors each year to a memorable weekday shared by dates like 4/4, 6/6, 8/8, 10/10 and 12/12. Our calculator does not need these shortcuts because it reads the date directly, but they are the same logic the tool automates.
One practical detail that trips people up is time zones. A weekday depends on the local date, not on Coordinated Universal Time. This tool reads the date at local midnight, so the answer always matches the calendar date you typed rather than a date shifted by your UTC offset. That matters most for dates near midnight and for birthdays recorded in a different country from where you now live.
Finally, remember that the Gregorian calendar was introduced in October 1582 and adopted at different times in different countries (Britain and its colonies switched in 1752, skipping 11 days). For any date after a countrys switch, the weekday this tool returns matches the historical record. For dates before the switch, the proleptic Gregorian weekday it shows may differ from the Julian date people used at the time.
When to use it
- Finding out what day of the week you, your child, or a famous person was born on.
- Checking which weekday a future date such as a wedding, birthday, deadline or holiday falls on.
- Working out the day of the year (ordinal date) for planning, logistics, shipping or scientific records.
- Looking up the ISO 8601 week number used in payroll runs, project schedules and spreadsheet reporting.
- Researching historical events to confirm the weekday a date fell on for articles, genealogy or trivia.
- Planning recurring events by seeing how a fixed date shifts weekday from one year to the next.
How to use the Day of the Week Calculator
- Enter or pick a date in the date field (it starts on today by default).
- Read the weekday name (Monday to Sunday) 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 in brackets when it differs from the calendar year.
- Change the date to compare any other past or future day instantly.
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)
Using Zellers congruence to find the weekday of 4 July 2026 by hand.
- Set q = 4 (day), m = 7 (July), and split the year: K = 26 (year in century), J = 20 (century).
- Compute the month term: floor(13 x (7+1) / 5) = floor(104/5) = 20.
- Add the parts: 4 + 20 + 26 + floor(26/4) + floor(20/4) + 5 x 20 = 4 + 20 + 26 + 6 + 5 + 100 = 161.
- Reduce modulo 7: 161 mod 7 = 0, and in Zellers scheme h = 0 means Saturday.
- Cross-check with the calendar: 4 July 2026 is indeed a Saturday.
Result: Saturday (h = 0 under Zellers congruence)
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 |
| 1 March 2028 | Wednesday | 2028 is a leap year, shift of 2 days |
Day of the year for the 1st of each month (common year vs leap year)
| Month (1st) | Common year | Leap year |
|---|---|---|
| January | 1 | 1 |
| February | 32 | 32 |
| March | 60 | 61 |
| July | 182 | 183 |
| October | 274 | 275 |
| December | 335 | 336 |
Doomsday rule anchor dates that all share the same weekday within a year
| Month | Doomsday date | Memory aid |
|---|---|---|
| April | 4/4 | Even month, day equals month |
| June | 6/6 | Even month, day equals month |
| August | 8/8 | Even month, day equals month |
| May | 9/5 | 9 to 5 at the 7-Eleven |
| July | 11/7 | 9 to 5 at the 7-Eleven |
| November | 7/11 | 9 to 5 at the 7-Eleven |
Common mistakes to avoid
- Confusing the ISO week number with a simple calendar-week count. The ISO 8601 week is not 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 date shifted by your UTC offset.
- 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.
- Applying a modern weekday to very old dates. Countries adopted the Gregorian calendar at different times (Britain in 1752, skipping 11 days). For dates before a countrys switch, the historical Julian weekday can differ from the Gregorian weekday shown here.
- Miscounting the day of the year across February. A common error is adding 28 for February in every year. In a leap year February has 29 days, so day-of-year totals for March onward are one higher than the common-year figures.
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 by Pope Gregory XIII in 1582.
- Zellers congruence
- A modular-arithmetic formula published by Christian Zeller in 1882 that computes the weekday of any date without a calendar.
- Doomsday rule
- A mental-math method by John Conway (1973) that anchors each year to a weekday shared by dates such as 4/4, 6/6 and 12/12.
- Ordinal date
- A date format that combines the year with the day of the year, for example 2026-201 for 20 July 2026.
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 also 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.
How can I calculate the day of the week in my head?
Two well-known methods work by hand. Zellers congruence uses a formula reduced modulo 7, while Conways Doomsday rule anchors each year to a shared weekday for dates like 4/4, 6/6 and 12/12. Both give the same answer this tool reads directly from the calendar.
What day of the week will my birthday fall on next year?
A fixed date usually shifts forward one weekday each year, or two weekdays if a 29 February falls between the two birthdays. Enter next years date to see the exact weekday without doing the math yourself.
Why do some dates skip a weekday between years?
Because a leap year has 366 days, a date from March onward lands two weekdays later than the year before, rather than one. That extra shift is caused by the 29 February added in leap years.
Is the weekday the same everywhere in the world?
For a given calendar date the weekday is the same globally, since the Gregorian calendar is shared. The only difference is time zones: a moment near midnight can be two different dates in two places, and each date keeps its own weekday.
Sources
- ISO 8601 date and time format , International Organization for Standardization