Day of the Week Calculator
Find what day of the week any date from year 1 to 9999 falls on, with Zeller and Doomsday steps, a weekday counter and a Friday the 13th finder.
About this tool
Day of the Week Calculator
Find what day of the week any date from year 1 to 9999 falls on, with Zeller and Doomsday steps, a weekday counter and a Friday the 13th finder.
The Day of the Week Calculator tells you which weekday any date fell on or will fall on, from 1 January of year 1 to 31 December 9999. It uses the proleptic Gregorian calendar — today's leap-year rules applied to every year — and counts days with pure integer arithmetic rather than the browser's date object, so early years work too. The answer comes with the date's day of the year, ISO week, quarter and Julian Day Number, and the same weekday is worked out by hand with Zeller's congruence or Conway's Doomsday rule. Further tabs count how many of each weekday lie between two dates, list every Friday the 13th in a range of years, and show your birthday's weekday for the next ten years.
How to use Day of the Week Calculator
- 1
Open Day of the Week Calculator on AltFTool — it loads instantly in your browser.
- 2
In Find the day, pick the Day and Month and type a Year from 1 to 9999, or press Today.
- 3
Read the weekday, how many days ago or ahead it is, the ISO week and day of the year, then switch between Zeller and Doomsday to see the working.
- 4
Open Count weekdays, set a Start date and End date, choose the Weekday to count and tick Include the end date if needed.
- 5
Use Date finder (for example Friday the 13th from 2026 to 2030) or Birthday to list matching dates, then press Copy list.
Why use Day of the Week Calculator
Any year from 1 to 9999
Day counting uses integer maths over the 400-year Gregorian cycle, so ancient and far-future dates are handled the same way as this year's.
Learn the method
Every result shows Zeller's congruence and the Doomsday rule step by step with your date's numbers, so you can check it or learn to do it in your head.
Count and find weekdays
Count every weekday between two dates with a Mon–Fri and weekend total, or list every date that matches a weekday, day of month and month.
Remembers your inputs
Your last dates and settings are kept in this browser's local storage so they are there when you come back; nothing is uploaded.
Doomsday dates for each month
| Month | Doomsday date | Memory aid |
|---|---|---|
| January | 3rd (4th in leap years) | 3 in 3 years, 4th in the 4th |
| February | 28th (29th in leap years) | Last day of February |
| March | 14th | Pi Day, 3/14 |
| April | 4th | 4/4 |
| May | 9th | 9-to-5 at 7-Eleven: 5/9 |
| June | 6th | 6/6 |
| July | 11th | 7/11 |
| August | 8th | 8/8 |
| September | 5th | 9/5 |
| October | 10th | 10/10 |
| November | 7th | 11/7 |
| December | 12th | 12/12 |
Century anchors: 1800s Friday, 1900s Wednesday, 2000s Tuesday, 2100s Sunday.
Common use cases
- Finding the weekday you were born on, or the day a historic event happened, such as the Moon landing on 20 July 1969.
- Counting how many Mondays, Saturdays or working days fall between two dates when planning classes, shifts or a payment schedule.
- Checking which months have a Friday the 13th, or which year a birthday or anniversary next lands on a weekend.
Frequently asked questions
What day of the week was I born on?
Choose your birth date in Find the day and the calculator shows the weekday immediately. The Birthday tab also shows it, along with the weekday of each of your next ten birthdays and how many of them fall on a Saturday or Sunday.
How do you calculate the day of the week for any date?
One method is Zeller's congruence: treat January and February as months 13 and 14 of the previous year, then h = (q + ⌊13(m + 1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7, where q is the day, K the last two digits of the year and J the century. h = 0 means Saturday, 1 Sunday, and so on.
What is the Doomsday rule?
The Doomsday rule, devised by John Conway, uses the fact that certain easy dates — 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7 and the last day of February — always share a weekday within a year. Find that year's doomsday from the century anchor and the last two digits, then count from the nearest of those dates.
Does it work for dates before 1582?
Yes, using the proleptic Gregorian calendar, which extends today's rules backwards. Countries switched from the Julian calendar at different times — Italy and Spain in 1582, Britain and its colonies in 1752, Russia in 1918 — so a date recorded in the Julian calendar before the switch can name a different weekday than the one shown here.
How many Mondays are there between two dates?
Open Count weekdays, enter the start and end dates and choose Monday. The count includes the start date, and the end date too when Include the end date is ticked. The table also shows every other weekday, the Monday-to-Friday total and the weekend total. Public holidays are not subtracted.
How often does Friday the 13th happen?
Every year has at least one Friday the 13th and never more than three. Over the 400-year Gregorian cycle the 13th falls on a Friday slightly more often than on any other weekday: 688 times, against 687 for Sunday and Wednesday, 685 for Monday and Tuesday, and 684 for Thursday and Saturday. The Date finder lists every one in a range of years.
Why does my birthday move forward one weekday each year?
A common year has 365 days, which is 52 weeks plus one day, so the same date moves one weekday later. When a 29 February comes between one birthday and the next, the year has 366 days and the birthday jumps two weekdays instead.
What day of the week was 1 January of year 1?
In the proleptic Gregorian calendar, 1 January 1 AD was a Monday. In the Julian calendar, which was actually in use in Rome at that time, the same day is usually given as a Saturday. There is no year 0 in this calculator: year 1 is the first year.
What is an ISO week number?
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday. So 1–3 January can belong to the last week of the previous year, and 29–31 December can belong to week 1 of the next. A year has 52 or 53 ISO weeks.
Does the calendar repeat?
Yes. The Gregorian calendar repeats exactly every 400 years, which is 146,097 days — a whole number of weeks. Individual years repeat more often: a common year's calendar comes round again after 6, 11 or 28 years, depending on where it falls relative to leap years.
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