About the Day of the Week Calculator
This day of the week calculator tells you which weekday any date falls on, from centuries ago to far in the future. Enter a date and you get the weekday, which occurrence of that weekday it is in the month (for example “the 4th Friday of December”), the ISO week number and the day of the year.
People use it to find out what day they were born on, check whether a holiday or deadline lands on a weekend, plan events years ahead, or verify dates in historical documents. The table shows the same calendar date across the surrounding years, which is useful for birthdays and anniversaries: it moves forward one weekday each year and two after a 29 February.
Dates use the Gregorian calendar, extended backwards for dates before it was adopted (the proleptic Gregorian calendar). Countries switched from the Julian calendar at different times — Britain and its colonies in 1752 — so very old dates recorded in the Julian calendar need converting first.
How to use the day of the week calculator
- 1Enter or pick the date you are curious about.
- 2Read the weekday and which occurrence of it that is in the month.
- 3Check the ISO week number and day of the year if you need them for planning.
- 4Scroll the table to see which weekday the same date falls on in other years.
Formula and method
Zeller’s congruence is the classic way to compute the weekday by hand. January and February are treated as months 13 and 14 of the previous year so the leap day falls at the end of the counting year. K is the year within the century and J the century; the result h gives 0 for Saturday, 1 for Sunday and so on.
The calculator gets the same answer by counting days from a fixed reference date in the proleptic Gregorian calendar and taking the remainder after dividing by 7. A handy mental shortcut is John Conway’s Doomsday rule: dates such as 4/4, 6/6, 8/8, 10/10, 12/12 and the last day of February all share one weekday each year (Saturday in 2026).
- q
- Day of the month
- m
- Month (3 = March … 14 = February)
- K
- Year of the century (year mod 100)
- J
- Zero-based century (⌊year ÷ 100⌋)
Worked examples
Independence Day 2026
4 July 2026 is a Saturday — the first Saturday of the month and day 185 of the year. Because it falls on a Saturday, the US federal holiday is observed on Friday 3 July.
Apollo 11 Moon landing
Neil Armstrong stepped onto the Moon on Sunday 20 July 1969 (US time), the third Sunday of that July and the 201st day of the year.
Christmas Day 2026
Christmas 2026 falls on a Friday, the fourth and last Friday of December, giving a three-day weekend. It is in ISO week 52.
New Year’s Day 2000
1 January 2000 was a Saturday. Because the ISO week containing it started on Monday 27 December 1999, that day belongs to week 52 of 1999.
Frequently asked questions
How can I find out what day of the week I was born?+
Enter your date of birth above and the calculator shows the weekday instantly. For example, anyone born on 1 January 2000 was born on a Saturday.
Why does my birthday move by one weekday each year?+
A common year has 365 days, which is 52 weeks plus 1 day, so a date shifts one weekday later each year. After a 29 February it shifts two days, because a leap year has 366 days.
How often does the calendar repeat?+
The full Gregorian calendar repeats every 400 years (146,097 days, exactly 20,871 weeks). Individual years reuse an earlier calendar after 6, 11 or 28 years depending on leap years.
What is the Doomsday rule?+
A mental method by John Conway. Each year, 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7 and the last day of February fall on the same weekday — the year’s “doomsday”. In 2026 it is Saturday.
Does this work for dates before 1582?+
Yes, using the proleptic Gregorian calendar. Historical documents from before a country adopted the Gregorian calendar usually use Julian-calendar dates, which differ from Gregorian ones by up to 13 days (10 days in 1582, 13 days since 1900), so convert them first.