How this calculator works
Enter any date — a birthday, a historical event, a future plan — and this calculator tells you which day of the week it falls (or fell) on, along with a few related facts: its position in the year (day-of-year), how many days that year has in total, and whether it is a leap year.
The weekday repeats on an exact 7-day cycle, so the same date in the same month always shifts weekday by one each common year and by two across a leap year. That is the whole mechanism behind trivia like "my birthday falls on a Tuesday this year" — no calendar lookup is really needed once you know one anchor date and count forward.
How the weekday and year facts are found
Weekday: count the days since a known reference date and take the remainder after dividing by 7 (every civil calendar system reduces to this).Day of the year: the number of days from January 1 of that year up to and including the given date.Leap year: true if the year is divisible by 4, unless it is also divisible by 100 — in which case it must also be divisible by 400.The leap-year rule is why 2000 was a leap year (divisible by 400) but 1900 and 2100 are not (divisible by 100 but not 400) — a detail that trips up naive "divisible by 4" checks.
Worked example
- Date: March 1, 2024.
- 2024 is a leap year (divisible by 4, not a century year), so January has 31 days and February has 29.
- Day of the year: 31 + 29 + 1 = 61.
- Counting the 7-day cycle from known reference dates gives Friday, and the year has 366 days total.
Frequently asked questions
Why does the same date fall on a different weekday every year?
A common year has 365 days, which is 52 weeks plus 1 extra day, so the weekday for any fixed date shifts forward by one day each common year. A leap year adds an extra day, so the shift is two days across a year that contains February 29.
How can I quickly check if a year is a leap year?
Divide the year by 4 — if it doesn't divide evenly, it isn't a leap year. If it does, check whether it's also divisible by 100; if so, it's only a leap year if it's additionally divisible by 400. That is why 2000 was a leap year but 1900 and 2100 are not.
What is day-of-year used for?
It's the count of days from the start of the year, sometimes called an ordinal date. Programmers, foresters and some scientific and military date formats use it directly (for example, "day 200" instead of "July 19").
Is this the same as the ISO week number?
No — day-of-year counts individual days from January 1. The Week Number Calculator instead groups dates into ISO 8601 weeks, which can belong to a different "week-year" than the calendar year near January 1 and December 31.
If two dates fall on the same day of the week, how many days apart must they be?
A multiple of 7 - 7, 14, 21, and so on (or 0, if they're the same date). Since the weekday repeats on an exact 7-day cycle, any two dates that many days apart always land on the same weekday, while a gap that isn't a multiple of 7 always lands on a different one.