Day of the Week Calculator
Pick any date from year 1 to 9999 and see which day of the week it falls on, how the answer is worked out, and the next years it lands on that day again.
This calculator gives the day of the week for any date from year 1 to 9999. 20 July 1969, the day of the first Moon landing, was a Sunday, and the same date next fell on a Sunday in 1975. The page also shows Zeller's formula worked out for your date, so you can check the answer by hand.
This calculator is for general reference. It uses the Gregorian calendar for all years and does not convert dates from the Julian calendar or any other calendar, so check any historical date against the original record.
Day of the week
Sunday
- ISO weekday number (Monday = 1)
- 7
- Next years on a Sunday
- 1975, 1980, 1986, 1997, 2003
Zeller's congruence (January and February count as months 13 and 14 of the year before): h = (20 + 20 + 69 + 17 + 4 + 95) mod 7 = 225 mod 7 = 1, which is Sunday.
Uses the Gregorian calendar for every year, including dates before it was adopted.
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About the Day of the Week Calculator
A date's weekday moves forward by one each year, because 365 days is 52 weeks and one day, and by two when a 29 February falls between the two dates. That is why the next matching year is usually 5, 6 or 11 years away, and 7 or 12 around a century year such as 2100 that skips its leap day. The calculator works the answer two ways and checks they agree: by counting days from a fixed starting point, and with the formula Christian Zeller published in 1887. In Zeller's method January and February count as months 13 and 14 of the year before, so 1 January 2000, a Saturday, goes into the formula as day 1 of month 13 in 1999. A 29 February exists only in leap years, so 29 February 2024, a Thursday, next falls on a Thursday on 29 February 2052, 28 years later. The Gregorian pattern of leap years repeats every 400 years, and 400 years hold a whole number of weeks, so every date has the same weekday as the date 400 years earlier.
Frequently asked questions
What day of the week was I born on?+
Enter your date of birth and read the large result. The list under it gives the next years your birthday falls on the same weekday, which is a handy way to plan a party on a Saturday.
How do you work out the day of the week by hand?+
Zeller's formula adds the day of the month, a month term worked out from the month number, the last two digits of the year, those two digits divided by 4 and rounded down, the century divided by 4 and rounded down, and five times the century, then takes the remainder after dividing by 7. A remainder of 0 means Saturday, 1 Sunday and so on up to 6 for Friday. The page shows every number for the date you enter.
Why does a historical date show a different weekday from old records?+
The calculator uses the Gregorian calendar for every year, even before it existed. Many Catholic countries switched from the older Julian calendar in 1582, and Britain and its colonies in 1752, when 2 September was followed by 14 September. A record written in the Julian calendar uses different date numbers, so check which calendar the source used.
What is the ISO weekday number?+
It is the international standard numbering used in software and week planners, where Monday is 1 and Sunday is 7. Some systems instead count Sunday as 0 or 1, so check which rule a spreadsheet or program expects.
Method: Chr. Zeller, "Kalender-Formeln", Acta Mathematica 9 (1887), pages 131 to 136. The calculator checks each result against the Julian Day Number method of H. F. Fliegel and T. C. Van Flandern, Communications of the ACM 11(10), 1968, page 657.
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