The formula
How it works
Find out which day of the week any date falls on — past or future. Enter a date and the calculator names the weekday, and tells you the day of the year, the week number, and whether it is a leap year.
FAQ
How can it know the weekday for any date?
The days of the week repeat in a perfect seven-day cycle, so once you know the weekday of one reference date, you can count forward or backward. The calculator does this with the built-in calendar, which already accounts for leap years.
Does it work for dates far in the past?
Yes, within the range of the standard calendar. Bear in mind that different countries switched from the Julian to the Gregorian calendar at different times, which can shift weekdays for very old dates.
Does it work for future dates too?
Yes — the same seven-day cycle runs forward indefinitely, so you can look up the weekday for a date decades from now just as easily as one from the past.
What is the week number it shows?
It is the week of the year your date falls in, counted from the 1st of January, which is handy for scheduling, invoicing, or matching up with a work calendar.
How is the day of the year calculated?
It counts every day from 1 January up to and including your chosen date, so 1 February is day 32 and the total reaches 365 or 366 depending on whether it is a leap year.
Why does it matter whether the year is a leap year?
A leap year adds 29 February, which shifts the weekday of every later date in that year by one day compared with a non-leap year — that extra day is exactly why the weekday of a fixed date changes from year to year.
Can I use this to check recurring dates, like anniversaries?
Yes — enter each year’s date to see how the weekday of a birthday, anniversary, or holiday shifts from one year to the next.
About the day of the week calculator
This calculator tells you which day of the week a given date falls on, for any date past or future. It is the tool for questions like “what day was I born on?” or “what weekday is Christmas next year?”. Alongside the weekday it reports the day’s position in the year, its week number, and whether the year is a leap year — a quick, complete snapshot of where a date sits in the calendar.
How to use it
Pick a date with the date picker. The calculator instantly names the day of the week and adds a few useful facts: which numbered day of the year it is, which week of the year, and whether it falls in a leap year. For example, 1 January 2000 was a Saturday, the first day of the year, in a leap year. Change the date and everything updates at once, so you can check birthdays, holidays or historical dates in seconds.
The formula
Because the weekday follows a strict seven-day cycle, it can be found from the number of days between your date and a known reference, taken modulo 7: . The calculator uses the calendar’s own day count, which already handles the 365- or 366-day years, so leap days are built in. The day-of-year and week-number come from counting days from the 1st of January.
Where it is used
People use it out of curiosity — the day they were born, married, or when a memorable event happened — and for planning, to see which weekday a future date lands on before booking travel or scheduling an event. Historians and genealogists use it to check records, and it is a classic programming and mental-maths puzzle. Knowing the weekday is often the first thing you want when a date is more than a few weeks away.