Day of the Week Calculator
What day any date falls on, past or future.
Work out Day of the Week. What day any date falls on, past or future. Built to be checked against a textbook.
Day of the week
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How the Day of the Week Calculator works
The weekday for any Gregorian date, computed by Zeller's congruence rather than looked up. It also gives the day of the year, the days remaining, whether the year is a leap year and the ISO week. The 20th of July 1969 was a Sunday; the 1st of January 2000 was a Saturday.
Also known as: what day was I born · what day of the week is this date · day of the year calculator · date to weekday calculator
The calculation itself
Zeller's congruence, which computes the weekday arithmetically rather than counting from a reference date.
The formula treats March as the first month of the year, which is why January and February are entered as months 13 and 14 of the preceding year. Shifting the calendar this way moves the leap day to the end, where an irregular February no longer disturbs the pattern the formula depends on.
In practice
The 20th of July 1969 was a Sunday. The 1st of January 2000 was a Saturday. The 29th of February 2024 was a Thursday.
The page also gives the day of the year, which runs to 365 or 366, and the days remaining. The 31st of December 2024 was day 366; the 31st of December 2026 is day 365.
Leap years are those divisible by 4, except centuries not divisible by 400. 2000 qualified on the exception to the exception; 1900 did not.
Why weekdays drift
365 divided by 7 leaves a remainder of one. A given date therefore advances one weekday each ordinary year, and two across a leap year.
The whole cycle repeats every 400 years, which is the length of the Gregorian leap rule. That period contains exactly 146,097 days, and 146,097 divides evenly by 7 — a deliberate feature of the calendar, and the reason the pattern of weekdays is perfectly periodic rather than drifting forever.
A caveat on very old dates
This computes the proleptic Gregorian calendar: the modern calendar extended backwards past its own adoption in 1582.
Historical records from before adoption used the Julian calendar, and countries switched at different times — Britain and its colonies not until 1752. A date from that era can have two legitimate weekdays depending on which calendar the source was using, so historical work needs more care than this page can supply.
Frequently asked questions
What day of the week was I born?
Enter your date of birth above. The calculation is exact for any Gregorian date, so it works equally for dates centuries back or forward.
How does the calculation work?
By Zeller's congruence, a formula that treats March as the first month of the year. That is why January and February are counted as months 13 and 14 of the previous year — shifting the calendar puts the leap day at the end, where it does not disturb the pattern.
Which years are leap years?
Those divisible by 4, except centuries not divisible by 400. 2000 was a leap year; 1900 was not; 2024 was and 2026 is not.
What is the day of the year?
The date's position counting from 1 January. The 31st of December is day 365, or day 366 in a leap year.
Does this work for dates before 1582?
It computes the proleptic Gregorian date — the Gregorian calendar extended backwards. Historical records before the calendar's adoption used the Julian calendar, and adoption dates differed by country, so historical dates that far back need care.
Why does the weekday of a date shift each year?
Because 365 is not divisible by 7 — it leaves a remainder of one. A date moves forward one weekday each year, and two across a leap year.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/day-of-week-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Day of the Week Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.