What this calculator does
Find out which day of the week any date falls on. Maybe you want the day you were born, or the weekday of a future wedding or deadline. Or maybe you wonder whether a historic event happened on a weekend. The question is the same: which of the seven days is it? The calendar repeats every 400 years. So every date has a fixed weekday you can work out with a little arithmetic.
Pick a date, and the calculator names its weekday. It tells you which one of that weekday it is in its month. That's handy for holidays set as "the fourth Thursday of November". It also says whether it's a weekday or a weekend day, and gives its day number in the year. The steps use Zeller's congruence, a method you can follow with pencil and paper.
Where it's used
Why this math matters outside the classroom:
- Birthday trivia. People enter a birth date such as July 20, 1969 and learn it was a Sunday.
- Holiday dates. Planners find the fourth Thursday of November 2026, which is November 26, by checking weekdays.
- Event planning. Couples check whether a chosen wedding date such as June 12, 2027 lands on a Saturday.
- History lessons. Students confirm that July 4, 1776 fell on a Thursday using Zeller's remainder method.
- Mental math. Puzzle fans divide a running total such as 183 by 7 and read the remainder 1 as Sunday.
How to use it
- Pick the date in the date field. It starts on today.
- Read the weekday. Below it, see which one of that weekday it is in the month and its day of the year.
- Open the steps to see Zeller's method worked on your date. It ends with a remainder from 0 to 6 that names the day.
What the inputs mean
- Date
- Any date from January 1 of year 1 to December 31, 9999. Our calendar today is the Gregorian calendar. It was adopted in 1582 in much of Europe, and in 1752 in Britain and its colonies. Earlier dates are given on the Gregorian calendar run backward. They're not on the older Julian calendar people used at the time.
Formula and method
h = (q + ⌊13(m + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5J) mod 7 q = day of the month; m = month (March = 3 … December = 12, January = 13, February = 14) K = year mod 100; J = ⌊year ÷ 100⌋ (using the previous year for January and February) h: 0 = Saturday, 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday, 6 = Friday
In the formula, q is the day of the month and m is the month number. K is the last two digits of the year, and J is the century (the first two digits). The brackets ⌊ ⌋ mean "round down to a whole number". "mod 7" means the remainder after dividing by 7. h is that remainder, which names the day.
Zeller's congruence adds up how far each part of the date pushes the weekday along. Each ordinary year moves the weekday forward by one (365 = 52 × 7 + 1). Each leap year moves it by two. Each month moves it by however many days it runs past four whole weeks. The term ⌊13(m + 1) ÷ 5⌋ packs all those month shifts into one expression.
January and February count as months 13 and 14 of the year before. That puts February 29 at the very end of the counting year. So the leap-year terms ⌊K ÷ 4⌋ and ⌊J ÷ 4⌋ only kick in from March onward, exactly when the extra day starts to matter.
Only the remainder after dividing by 7 matters, because the weekdays repeat every 7 days. The calculator checks the result against a plain count of days from a known Thursday (January 1, 1970). So both methods always agree.
Worked example
What day of the week is October 18, 2026?
- October is month m = 10, so the year stays 2026: K = 26 and J = 20, with q = 18.
- ⌊13 × 11 ÷ 5⌋ = ⌊28.6⌋ = 28, ⌊26 ÷ 4⌋ = 6, ⌊20 ÷ 4⌋ = 5 and 5 × 20 = 100.
- Sum: 18 + 28 + 26 + 6 + 5 + 100 = 183.
- 183 ÷ 7 = 26 remainder 1, and a remainder of 1 means Sunday.
Result: October 18, 2026 is a Sunday, the 3rd Sunday of October and day 291 of the year.
Understanding the result
"Last" means no later date in that month falls on the same weekday. That's how holidays such as the last Monday of May are set.
Weekend means Saturday or Sunday. Some countries have a Friday–Saturday weekend, so check local practice for work schedules.
A date and the same date 28 years later usually share a weekday. But a century year that isn't a leap year (such as 2100) breaks that pattern.
Assumptions and limitations
- All dates are on the Gregorian calendar, extended backward before 1582.
- The answer is a calendar fact. It doesn't depend on your time zone.
- Old dates written on the Julian calendar need converting first. Most English records before September 1752 are an example. Only then will the weekday match what people wrote at the time.
- Years before 1 and after 9999 aren't supported.
Frequently asked questions
How do I find what day of the week I was born?
Enter your birth date. The calculator gives the weekday right away. The steps show how to check it by hand with Zeller's method.
Why are January and February treated as months of the previous year?
It moves the leap day to the end of the counting year. That way the leap-year fix only affects dates after February 29, where the extra day really shifts the weekday.
Does a date fall on the same weekday every year?
No. A date moves forward one weekday each year, or two when a February 29 falls in between. October 18 is a Sunday in 2026, a Monday in 2027 and a Wednesday in 2028.
Why does an old document give a different weekday?
Many countries used the Julian calendar before switching to the Gregorian one. Over time the two drifted up to 10 or more days apart. So a date written under the old calendar means a different day than the same numbers on the Gregorian calendar.
How this calculator is checked
Method: Zeller's congruence for the Gregorian calendar, cross-checked against a day count from Thursday, January 1, 1970. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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