Day of Week Calculator
Find what day of the week any date was or will be: day of the year, ISO 8601 week number, leap year check. The calculator works online and is free.
Why look up the day of the week for a date
"What day of the week was I born?", which weekday a birthday or vacation will fall on, what day New Year 1980 was — questions like these come up all the time: filling in forms, exploring a family archive or planning events. Counting by hand with Zeller's congruence is slow and easy to get wrong. Pick a date in the calculator and it shows the day of the week, the day of the year, the ISO 8601 week number and whether the year is a leap year — plus a set of famous historical dates one click away.
What it does
- Day of the week for any date — past or future, in English or Russian.
- Day of the year: the ordinal day number out of 365 or 366.
- Week number per the ISO 8601 standard — the one used in business planning and reports.
- Leap year check according to the Gregorian calendar.
- Bonus — historical dates: Gagarin's flight, the Moon landing, Victory Day and the millennium in one click.
How to use it
- Pick a date in the field — for example, your date of birth or a famous historical event.
- Read the result: the day of the week in large type, then the day of the year, the ISO 8601 week number and the leap year check.
- Click "Copy result" to paste the answer into a document or a message.
How it works
The date from the input field is parsed as a calendar day with no time component, so the timezone cannot shift the result. The weekday is determined through the standard Date object and the browser's Intl.DateTimeFormat, which knows the calendar centuries ahead and back. The week number follows ISO 8601: weeks start on Monday, and week 1 is the one containing the first Thursday of the year. Leap years follow the Gregorian rule: divisible by 4, except centuries, unless divisible by 400. Everything runs locally in your browser.
Interesting historical dates
| Date | Day of the week | Event |
|---|---|---|
| April 12, 1961 | Wednesday | first human spaceflight |
| July 20, 1969 | Sunday | first human step on the Moon |
| May 9, 1945 | Wednesday | Victory Day |
| January 1, 2000 | Saturday | the millennium |
| December 31, 1999 | Friday | last day of the 20th century |
| February 29, 2024 | Thursday | leap day |
Frequently asked questions
How do I find out what day of the week I was born?
Enter your date of birth in the calculator field and the weekday appears right away in large type. For example, people born on January 1, 2000 were born on a Saturday. You will also see your ordinal day of the year: those born on December 31 arrived on day 365 (or 366).
What day of the week was April 12, 1961?
It was a Wednesday. On April 12, 1961 Yuri Gagarin made the first human spaceflight — an ordinary working day of the week. Pick that date in the calculator (it is in the "Historical dates" block) and see for yourself.
Why was 2000 a leap year but 1900 was not?
Under the Gregorian century rule a year divisible by 4 is a leap year, but years divisible by 100 are ordinary — unless they are also divisible by 400. The year 1900 divides by 100 but not by 400, so it was not a leap year. The year 2000 divides by both 100 and 400, so it had 366 days. Without this rule the calendar would drift about a day every 128 years.
What is an ISO 8601 week number?
It is the international standard for numbering weeks: a week starts on Monday, and week 1 is the week that contains the first Thursday of the year. Because of that, early January can belong to the last week of the previous year: for example, January 1, 2000 (a Saturday) was part of week 52 of 1999. Reporting and planning systems use this numbering.
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