Day of the Week Finder

What day of the week was it?

This day of the week finder tells you the weekday of any calendar date, past or future. Enter a month, a day and a year, and you see whether it was or will be a Monday or a Saturday, together with the day of the year, the ISO week number and how many days the date is from today. It answers everyday questions such as which weekday you were born on, whether a birthday or a wedding date falls on a weekend, or which day of the week a date in a letter or a diary was.

For a birthday, the table under the result lists the same date in twelve years around today, and you can page back and forward through the years. To count the days between two dates instead, use days between dates. For an exact age in years, months and days, use the age calculator, and for both the ISO and the US week number of a date, the week number calculator.

How the day of the week is calculated

The finder uses Zeller's congruence, a short formula that turns a date into a number from 0 to 6. It needs four inputs. q is the day of the month. m is the month, with one twist: January and February count as months 13 and 14 of the previous year, so the leap day sits at the very end of the counting year. K is the year within its century, the last two digits, and J is the century, the year divided by 100 and rounded down. For the Gregorian calendar:

The result h stands for the weekday, starting with Saturday: 0 is Saturday, 1 is Sunday, 2 is Monday and so on up to 6 for Friday. Take July 4, 1776. Here q = 4, m = 7, K = 76 and J = 17. The terms are 4, ⌊13 × 8 / 5⌋ = 20, 76, ⌊76 / 4⌋ = 19, ⌊17 / 4⌋ = 4 and 5 × 17 = 85. They add up to 208, and 208 mod 7 is 5, so July 4, 1776 was a Thursday. The step by step box above runs the same calculation for your date, with every number shown.

The term ⌊13(m + 1) / 5⌋ accounts for the uneven lengths of the months, K and ⌊K / 4⌋ add one weekday per year plus one per leap year, and the J terms handle the century rule. The finder checks its answer against a second method that counts the days from a fixed starting point, so both always agree. For the Julian calendar, the last two terms are replaced by 5 + 6J, because that calendar has no century rule.

Gregorian and Julian calendars

The Gregorian calendar is the one in use today. In it, a year divisible by 4 is a leap year, except for century years, which are leap years only when they are divisible by 400. So 1900 was not a leap year, but 2000 was. The older Julian calendar makes every fourth year a leap year without exception, which is slightly too long, and over the centuries the calendar drifted away from the seasons.

Pope Gregory XIII introduced the reform in 1582. Ten days were dropped, so Thursday, October 4, 1582 was followed by Friday, October 15, 1582. Catholic countries adopted it first. Protestant states first rejected the new calendar and then accepted it one by one over the following centuries. Britain and its colonies switched in 1752 by an act of Parliament: the day after September 2, 1752 became September 14. Eastern Orthodox churches still use the Julian calendar to set the date of Easter.

The weekday cycle was not interrupted by these switches; only the dates were renamed. That is why the finder can give you the weekday of any day in either calendar. With the Gregorian setting, dates before October 15, 1582 are in the proleptic Gregorian calendar, the Gregorian rules extended back in time, so old and modern dates sit on one scale. The note under the result shows the same day in the Julian calendar, and the gap between the two grows by one day each century year that is not divisible by 400:

If your date comes from an old document, check which calendar the writer used. In England and the British colonies the Julian calendar was the official one until September 2, 1752, and before October 15, 1582 the Gregorian calendar did not exist yet.

Years BC and the missing year zero

In the usual way of writing years, 1 BC is followed directly by AD 1, with no year 0 in between. That makes arithmetic across the boundary awkward, so astronomers count 1 BC as year 0, 2 BC as year −1 and 44 BC as year −43. The finder works the same way internally and shows that number in the step by step view. Remember that both calendars are applied by their rules to every year: the Julian calendar was introduced in 45 BC, so any earlier date is a calculation rather than a date anyone used.

Why a birthday moves through the week

A common year has 365 days, which is 52 weeks and one day. So a date moves one weekday later each year: a birthday on a Monday this year is on a Tuesday next year. When a February 29 falls between the two dates, it moves two weekdays. That is why the birthday table never shows the same weekday two years in a row. Someone born on February 29 only has that date in leap years, and the table marks the other years. The Gregorian calendar repeats exactly every 400 years, because 400 years have 146,097 days, which is a whole number of weeks.

How to use the day of the week finder

1. 1Choose the month and type the day and the year. For a date before AD 1, pick BC, and for a date from an old document, consider switching the calendar to Julian.
2. 2Read the weekday at the top of the result, with the day of the year, the ISO week and how many days the date is from today.
3. 3Open the step by step box to see Zeller's congruence with your numbers, page through the birthday table, then copy the result or a link to the date.

Built by Tobias Kurth, designer. Updated October 13, 2026.

Frequently asked questions