Find the day of the week for any date
Were you born on a Sunday? What day was 4 July 1776? Will your wedding date fall on a weekend? This day of the week calculator answers instantly for any date from the year 1 to 9999 and, unlike a plain lookup, shows you how the answer is worked out using two classic methods you can do in your head or on paper: John Conway’s Doomsday rule and Zeller’s congruence.
A second tool on the same page finds every date that matches a pattern — every Friday the 13th between 2026 and 2030, every Monday in February, or every year your birthday lands on a Saturday. Everything runs in your browser using exact integer calendar arithmetic, so there are no time-zone or daylight-saving surprises.
How to use the Day of the Week Calculator
- Pick a date in the date field, press “Today”, or tap one of the example dates.
- Read the weekday in the result box, with the date written out in full so there is no day/month confusion.
- Check the extra details: which occurrence of that weekday it is in the month, the day of the year, the ISO week and how far it is from today.
- Switch between “Doomsday rule” and “Zeller’s congruence” to follow each method step by step with your date’s numbers.
- To list matching dates, choose a weekday, a day of the month and month (or “Any”/“Every”), plus a range of years, under “Find every date that falls on a weekday”.
Day of the Week Calculator features
Any date, years 1–9999
Works far into the past and future using the proleptic Gregorian calendar, with a note on the Julian date for older days.
Two worked methods
Every result shows the Doomsday rule and Zeller’s congruence with your own numbers filled in — ideal for students and puzzle fans.
Self-checking
The weekday is computed three independent ways (day count, Doomsday and Zeller), and they always agree.
Pattern finder
List all dates that fall on a chosen weekday and day of the month across up to 400 years, with a per-year count.
Copy the list
Copy matching dates in ISO format to paste into a calendar, spreadsheet or message.
The Doomsday rule, step by step
Mathematician John Conway noticed that certain easy-to-remember dates always share the same weekday within a year, which he called the year’s “doomsday”. They include 4/4, 6/6, 8/8, 10/10 and 12/12, plus 9/5, 5/9, 7/11 and 11/7 (“I work 9 to 5 at 7-Eleven”), 14 March (Pi Day) and the last day of February.
To find a weekday: start with the century anchor (Tuesday for the 2000s, Wednesday for the 1900s, Friday for the 1800s, Sunday for the 2100s). Add the number of dozens in the last two digits of the year, the remainder, and the number of fours in that remainder. The total, counted on from the anchor, gives the year’s doomsday. Finally, count forwards or backwards from the nearest doomsday date in your month.
Example for 7 October 2026: anchor Tuesday (2). 26 = 2 dozens remainder 2, and 2 contains no fours, so 2 + 2 + 2 + 0 = 6 → Saturday is the 2026 doomsday. 10 October is a Saturday, so 7 October, three days earlier, is a Wednesday.
| Month | Doomsday date | Month | Doomsday date |
|---|---|---|---|
| January | 3 (4 in leap years) | July | 11 |
| February | 28 (29 in leap years) | August | 8 |
| March | 14 | September | 5 |
| April | 4 | October | 10 |
| May | 9 | November | 7 |
| June | 6 | December | 12 |
Zeller’s congruence
The German mathematician Christian Zeller devised this formula in the 1880s. For the Gregorian calendar it is h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7, where q is the day of the month, m the month (March = 3 … December = 12, with January and February treated as months 13 and 14 of the previous year), K the year within the century and J the century. The result h runs from 0 = Saturday to 6 = Friday. Moving January and February to the end of the year puts the leap day last, which is what makes the formula so compact.
Julian and Gregorian calendars
The Gregorian calendar replaced the Julian calendar in Catholic countries in October 1582: Thursday 4 October was followed directly by Friday 15 October. The weekday cycle was never interrupted — only the date numbers jumped. Britain and its American colonies switched in 1752, Japan in 1873, Russia in 1918 and Greece in 1923, so historical dates from those countries may be written in the old style. For dates before 1582 this calculator uses the proleptic Gregorian calendar, as ISO 8601 does, and shows the Julian equivalent alongside. To see why the Gregorian leap-year rule matters, try the Leap Year Checker.
Friday the 13th and other patterns
Every year has at least one Friday the 13th and never more than three; 2026 has three (February, March and November). Over the full 400-year Gregorian cycle, the 13th of the month actually lands on a Friday slightly more often than on any other weekday — 688 times, against 684 to 687 for the others. Because 400 Gregorian years contain exactly 20,871 weeks, the whole calendar repeats every 400 years. To print a month with these dates marked, use the Calendar Generator, and to see which week a date sits in, the Week Number Calculator.
Day of the Week Calculator FAQ
What day of the week was I born?
Enter your date of birth in the day of the week calculator and the weekday appears immediately, along with the worked calculation. The Age Calculator will tell you your exact age as well.
How can I work out the day of the week in my head?
Learn the Doomsday rule: memorise the century anchor and the doomsday dates (4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 7/11, 11/7), then find the year’s doomsday with the dozens-remainder-fours trick. With practice it takes a few seconds.
Does it work for dates before 1582?
Yes. Earlier dates use the proleptic Gregorian calendar, and the tool shows what the date was called in the Julian calendar that people were actually using. The weekday is the same in both.
Why does the calendar repeat every 400 years?
A 400-year Gregorian cycle has 146,097 days, which is exactly 20,871 weeks, so every date falls on the same weekday as it did 400 years earlier.
How many Friday the 13ths are there in a year?
Between one and three. A year that starts on a Thursday (or a leap year that starts on a Sunday) has three; use the finder above to list them for any range of years.
Is this calculator accurate for future dates?
Yes. The Gregorian rules are fixed, so any date up to 31 December 9999 is calculated exactly.