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Day of the week calculator

Pick any date to see which day of the week it falls on, with the math shown so you can check it by hand.

By the Calcist team · Checked against known answers · Updated October 27, 2026

Private: calculated on your device. Nothing you type is sent anywhere.

Next: Want to count days between two dates? Use days between dates, or see how old someone born that day is with the age calculator.

How to work out the day of the week

This calculator shows Zeller's congruence, a formula from 1882 that gives the weekday for any Gregorian date:

h = (q + ⌊13(m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5J) mod 7
  • q is the day of the month.
  • m is the month, with March = 3 up to December = 12, and January and February counted as months 13 and 14 of the year before.
  • K is the year within the century (year mod 100) and J is the century (⌊year ÷ 100⌋), using that adjusted year.
  • h gives the day: 0 = Saturday, 1 = Sunday, 2 = Monday, … 6 = Friday.

Worked example: July 4, 1776

q = 4, m = 7, K = 76, J = 17.

h = (4 + ⌊13 × 8 ÷ 5⌋ + 76 + ⌊76 ÷ 4⌋ + ⌊17 ÷ 4⌋ + 5 × 17) mod 7
= (4 + 20 + 76 + 19 + 4 + 85) mod 7 = 208 mod 7 = 5 → Thursday

Why the calendar repeats

A common year is 52 weeks and 1 day, so each date moves one weekday later the next year, or two after February 29. The Gregorian calendar repeats exactly every 400 years (146,097 days, which is exactly 20,871 weeks), so January 1, 2000 and January 1, 2400 are both Saturdays.

Limitations

  • Old dates. The Gregorian calendar started on October 15, 1582 (Thursday, October 4 in the old Julian calendar was followed by Friday, October 15). Britain and its American colonies switched in 1752. Dates before the switch are often written in the Julian calendar. For those, the calculator also shows the weekday if the date is a Julian one.
  • Earlier dates here use the Gregorian calendar extended backwards (the "proleptic" Gregorian calendar, as in ISO 8601), so they're consistent but may not match a historical document.

Sources

How we checked this

Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:

  • July 4, 1776 was a Thursday ✓
  • January 1, 2000 was a Saturday ✓
  • January 1, year 1 (proleptic Gregorian) is a Monday ✓
  • October 15, 1582, the first Gregorian day, was a Friday ✓
  • Julian June 15, 1215 (Magna Carta) was a Monday ✓
  • Edge: Julian March 1, AD 1 was a Tuesday ✓

Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.

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Questions people ask

What day of the week was July 4, 1776?

A Thursday.

What day of the week was January 1, 2000?

A Saturday. So is January 1, 2400, because the calendar repeats every 400 years.

How can I work out the day of the week in my head?

Learn John Conway's Doomsday rule: in any year, 4/4, 6/6, 8/8, 10/10, 12/12 and the last day of February all fall on the same weekday. For 2026 that day is a Saturday.

What is the ISO week number?

ISO 8601 weeks start on Monday, and week 1 is the week with the year's first Thursday. Early January days can belong to the previous year's last week.

Why do old dates sometimes disagree with history books?

Many countries still used the Julian calendar then. George Washington was born on February 11, 1732 (Julian), which is February 22 in the Gregorian calendar. Both were a Friday.

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