What the Day of the Week Calculator does
This calculator tells you the day of the week for any date from the year 1 to 9999 - the day you were born, a historical event, a future deadline - and shows the working two ways: Zeller's congruence and John Conway's Doomsday rule. Type a day, month and year and the weekday appears at once; paste a list and every date is checked together.
Old dates need one extra decision: which calendar. Britain and its colonies used the Julian calendar until 2 September 1752, and most of Catholic Europe until 4 October 1582. Choose the historical calendar and the tool applies the switch for you, and refuses the dates that never existed.
How to use it
- Enter the day, choose the month and type the year. The result updates as you type; Today fills in today's date.
- Leave the calendar on Gregorian for anything after 1752. For older dates choose the British 1752 switch (England, Scotland, Ireland, colonial America) or the 1582 switch (Italy, Spain, Portugal, Poland); choose Julian throughout for Orthodox church dates.
- Read the weekday in the big panel and the two worked methods below it; both always agree with the Julian Day Number shown alongside.
- To check many dates, open "Check a list of dates", paste one per line, say whether numeric dates are day-first or month-first, and press the button. Unreadable lines and dates in a calendar gap get their own row with the reason. Download the table as CSV.
Reading the results
"Gregorian" means the proleptic Gregorian calendar: today's leap-year rules run backwards to every year. That is what ISO 8601, spreadsheets and most software use, so it is the right choice for matching another system, but it is not what anyone wrote before their country adopted it.
The "same day" figure converts between calendars. Shakespeare died on 23 April 1616 in England's Julian calendar, a Tuesday - which was 3 May in the Gregorian calendar. Cervantes died on Gregorian 23 April 1616, a Saturday: ten days apart despite the famous shared date.
Unlike the Date Calculator, which adds and subtracts days, this page answers the weekday question directly, shows the arithmetic, checks whole lists and handles the Julian-to-Gregorian change that ordinary date libraries ignore.
Worked example: the Moon landing, 20 July 1969
Zeller: q = 20, July is month m = 7, the year of the century K = 69 and the century J = 19. h = 20 + floor(13 x 8 / 5) + 69 + floor(69 / 4) + floor(19 / 4) + 5 x 19 = 20 + 20 + 69 + 17 + 4 + 95 = 225, and 225 mod 7 = 1. Zeller counts 0 = Saturday, so 1 = Sunday.
Doomsday: the 1900s anchor is Wednesday. For 69: 69 / 12 = 5 remainder 9, and 9 / 4 = 2, so Wednesday + 16 days = Friday. July's doomsday is the 11th, so 11 July 1969 was a Friday and the 20th, nine days later, a Sunday.
Both agree: Apollo 11 landed on Sunday 20 July 1969 (US and UK time; it was already Monday in India).
Formulas and scoring rules
- Zeller's congruence (Gregorian)
h = (q + floor(13(m + 1) / 5) + K + floor(K / 4) + floor(J / 4) + 5J) mod 7q = day; m = month with March = 3 ... February = 14, January and February counted in the previous year; K = year mod 100; J = floor(year / 100). h = 0 is Saturday, 1 Sunday ... 6 Friday.- Zeller's congruence (Julian)
h = (q + floor(13(m + 1) / 5) + K + floor(K / 4) + 5 + 6J) mod 7- Doomsday of a year
doomsday = (anchor + floor(y / 12) + (y mod 12) + floor((y mod 12) / 4)) mod 7y = last two digits. Gregorian anchor = (5 x (century mod 4) + 2) mod 7 with 0 = Sunday; Julian anchor = (-century) mod 7. Doomsdays: 3 Jan (4 in leap years), 28 Feb (29), 14 Mar, 4 Apr, 9 May, 6 Jun, 11 Jul, 8 Aug, 5 Sep, 10 Oct, 7 Nov, 12 Dec.- Julian Day Number
a = floor((14 - month) / 12); y = year + 4800 - a; m = month + 12a - 3; JDN = day + floor((153m + 2) / 5) + 365y + floor(y / 4) - floor(y / 100) + floor(y / 400) - 32045Julian calendar: ... + floor(y / 4) - 32083. Weekday = (JDN + 1) mod 7 with 0 = Sunday; 1 January 2000 is JDN 2451545.
The calendar changes of 1582 and 1752
The Julian calendar adds a leap day every four years, making its year 11 minutes too long. By 1582 the drift was ten days, so the Gregorian reform removed 5 to 14 October 1582 and dropped the leap day from century years not divisible by 400.
Great Britain changed under the Calendar (New Style) Act 1750: Wednesday 2 September 1752 was followed by Thursday 14 September - eleven days, because 1700 had been a Julian leap year. The weekdays ran on unbroken; only the day numbers skipped. Russia (1918) and Greece (1923) changed later still.
Limitations: what the result does not prove
- Only the 1582 and 1752 switches are built in. For countries that changed at other dates, use Julian before their switch and Gregorian after it.
- Years count from 1 January, from 1 to 9999. Before 1752 English documents often began the year on 25 March, so "11 February 1731" in a parish register is 1732 here. No BC dates.
- The weekday belongs to the calendar date, not a moment in a time zone: a late-night event may fall on the next day elsewhere.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- ISO 8601 - Date and time format - checked 19 Sep 2026
- Calendar (New Style) Act 1750 - legislation.gov.uk
- US Naval Observatory - Julian Date converter
Frequently asked questions
What day of the week was I born on?
Enter your date of birth and the weekday appears straight away, with the working. For anyone born since the 1920s the Gregorian calendar is right everywhere, so leave the calendar setting alone.
How do you work out the day of the week for any date by hand?
Use the Doomsday rule: start from the century anchor (Wednesday for the 1900s, Tuesday for the 2000s), add the years divided by 12, the remainder, and the remainder divided by 4, then count from the month's doomsday such as 4/4, 6/6, 8/8, 10/10 or 12/12.
What is Zeller's congruence?
A single formula, published by Christian Zeller in the 1880s, that turns a day, month and year into a number from 0 to 6 for the weekday. It treats January and February as months 13 and 14 of the previous year so the leap day falls at the end.
Why do some calculators give a different weekday for very old dates?
They apply today's Gregorian rules to years when the Julian calendar was in use. For an English date before 14 September 1752, or a Roman one before 15 October 1582, choose the historical calendar here to get the weekday people actually used.
Which days are missing from September 1752?
In Great Britain and its colonies 3 to 13 September 1752 never happened: Wednesday the 2nd was followed by Thursday the 14th. The tool refuses those dates under the British option and says why, rather than quietly shifting them.
Can I find the weekday for a whole list of dates at once?
Yes. Paste up to 1,000 dates, one per line, in formats such as 1969-07-20, 20 July 1969 or 20/07/1969, and choose day-first or month-first for numeric dates. You get a sortable table and a CSV download.
Is my date sent anywhere?
No. All the arithmetic runs in your browser and nothing you type leaves the page, so it is safe to check birthdays and other personal dates.
Last reviewed by the A2Z.Tools team against the sources listed above.
