By the Age-Finder team · Last updated 7 November 2026
Quick answer
Name any date from the year 1000 to 9999 and the calculator tells you its weekday: 20 July 1969 was a Sunday, and 25 December 2026 is a Friday. It also shows the day of the year, the ISO week number and whether the year is a leap year.
The Day of the Week Calculator tells you which weekday any date fell on or will fall on, from the year 1000 to 9999. It also shows the day of the year, the ISO week number and whether the year is a leap year.
How to Use the Day of the Week Calculator
Choose the day and the month, and type the 4-digit year. Select Calculate. The result shows the weekday, how long ago or how far ahead the date is, its day of the year, its ISO week number and the number of days in that month.
It also shows Zeller’s formula worked out, the same date in nearby years, and a calendar of the month. For a date written in the Julian calendar, tick “Julian calendar date” before you calculate, and the result switches to the Julian weekday.
For your own birthday, the Birthday Calculator shows the weekday of your next birthday, and the Date of Birth Calculator shows the weekday you were born on. For the weekday of your birthday across the years 2026 to 2030, see what day of the week is my birthday.
Weekdays of Famous Dates
| Date | Weekday | Event |
|---|---|---|
| 11 November 1918 | Monday | First World War armistice |
| 20 July 1969 | Sunday | Apollo 11 Moon landing |
| 9 November 1989 | Thursday | Fall of the Berlin Wall |
| 1 January 2000 | Saturday | Start of the year 2000 |
| 25 December 2026 | Friday | Christmas Day 2026 |
| 29 February 2028 | Tuesday | The next leap day |
How the Weekday Is Worked Out
Weekdays repeat in a cycle of seven, so the weekday of any date depends on how many days separate it from a date whose weekday is known. The calculator counts every day exactly, using real month lengths and every leap year. The Gregorian calendar repeats perfectly every 400 years.
Those 400 years contain 146,097 days, which is exactly 20,871 weeks, so 1 January 2400 falls on a Saturday, just like 1 January 2000. Within a single year, a date moves forward by one weekday each year, or by two when a 29 February falls in between.
Working it out by hand
You can check a result without a calculator. Start from a date you know, count the days to your date, and divide by 7. The remainder tells you how many weekdays to move forward.
For example, 1 January 2026 is a Thursday. Christmas Day 2026 is 358 days later. 358 divided by 7 is 51 weeks with a remainder of 1, so 25 December 2026 is one weekday after Thursday: a Friday.
Zeller’s Congruence, Worked by Hand
Christian Zeller published his congruence in the 1880s: a formula that turns any Gregorian date into a weekday with integer arithmetic alone. January and February count as months 13 and 14 of the previous year, which moves the leap day to the very end of the counting year.
With q the day of the month, m the shifted month, K the year within the century and J the century, the formula is h = (q + floor(13(m + 1) / 5) + K + floor(K / 4) + floor(J / 4) + 5J) mod 7,
Where h = 0 is Saturday, 1 is Sunday and 6 is Friday. The floor brackets drop the fraction.
Worked example: 20 July 1969. Here q = 20, m = 7, K = 69 and J = 19. The month term is floor(13 x 8 / 5) = 20, floor(69 / 4) = 17 and floor(19 / 4) = 4.
The sum is 20 + 20 + 69 + 17 + 4 + 95 = 225, and 225 mod 7 = 1, which is a Sunday. The same arithmetic gives 4 July 1776 as a Thursday: (4 + 20 + 76 + 19 + 4 + 85) mod 7 = 208 mod 7 = 5.
And 1 January 2000, which counts as month 13 of 1999: (1 + 36 + 99 + 24 + 4 + 95) mod 7 = 259 mod 7 = 0, a Saturday, and 259 is 7 x 37 exactly.
The Doomsday Rule, Worked by Hand
John Conway’s Doomsday rule finds any weekday mentally, once practised. It rests on one fact: in any year, a set of easy dates all share the same weekday, which Conway called the Doomsday.
The century anchor for the 2000s is Tuesday. Take the last two digits of the year, divide by 12 and note the quotient and the remainder, divide the remainder by 4, and add all three to the anchor, mod 7.
For 2026: 26 divided by 12 is 2 remainder 2, and 2 divided by 4 is 0, so Tuesday + 2 + 2 + 0 is a Saturday. That is the Doomsday of 2026.
| Month | Doomsday date | Memory aid |
|---|---|---|
| January | 3rd (4th in leap years) | 3 for 3 years in 4 |
| February | 28th (29th in leap years) | The last day of the month |
| March | 14th | Pi day |
| April | 4th | 4/4 |
| May | 9th | 9-to-5 at 7-11 |
| June | 6th | 6/6 |
| July | 11th | 7-11 |
| August | 8th | 8/8 |
| September | 5th | 9-to-5 at 7-11 |
| October | 10th | 10/10 |
| November | 7th | 7-11 |
| December | 12th | 12/12 |
Worked example: 30 August 2026. August’s Doomsday date is the 8th, a Saturday. 30 minus 8 is 22 days, and 22 mod 7 is 1, so one day past Saturday: 30 August 2026 is a Sunday.
When Calendars Repeat
Between 1901 and 2099, the calendar repeats exactly every 28 years: every date falls on the same weekday as it did 28 years earlier. A year can also share its whole calendar with a nearer year.
The 2026 calendar, which starts on a Thursday and is not a leap year, is used again in 2037. The 28-year pattern breaks around century years that are not leap years, such as 1900 and 2100, which is why the full Gregorian cycle is 400 years long.
Day of the Year
The day of the year counts days from 1 January, which is day 1. In a common year, 31 December is day 365; in a leap year it is day 366.
Christmas Day 2026 is day 359. Day-of-year numbers are used in logistics, manufacturing date codes, astronomy and data work, where a single number is easier to sort than a full date.
ISO Week Numbers
The ISO week number comes from the international standard ISO 8601. Its rules are: Weeks start on Monday. Week 1 is the week that contains the year’s first Thursday. Most years have 52 weeks, but some have 53.
Because of the Thursday rule, the first and last days of a year can belong to a week of the neighbouring year. For example, 1 January 2000 was a Saturday, so it belongs to week 52 of 1999.
Between 2020 and 2032, the years with a week 53 are 2020, 2026 and 2032. The calculator shows the week-year in brackets whenever it differs from the calendar year.
In Excel and Google Sheets
With the date in A1:
| What you want | Formula | Notes |
|---|---|---|
| The weekday name | =TEXT(A1,"dddd") |
Returns Monday, Tuesday and so on. |
| The ISO weekday number | =WEEKDAY(A1,2) |
Monday is 1 and Sunday is 7, as in ISO 8601. |
| The ISO week number | =ISOWEEKNUM(A1) |
Excel 2013 and later, and Google Sheets. |
| The ISO week number, older Excel | =WEEKNUM(A1,21) |
The 21 means ISO weeks, Monday start. |
| The day of the year | =A1-DATE(YEAR(A1),1,1)+1 |
Counts from 1 January as day 1. |
Dates Before 1582
The calculator uses the Gregorian calendar for every date, including dates before it was introduced. This is called the proleptic Gregorian calendar, and it is the system used by ISO 8601 and most software. Many countries used the older Julian calendar until 1582 or later.
Britain and its colonies switched in 1752, and Russia in 1918. Historical records written in the Julian calendar can therefore show a different date, and a different weekday, from the Gregorian date shown here. For historical research, check which calendar the original source used.
What other day of the week calculators offer
Most day of the week calculators on the first page give the same weekday this one does. What sets them apart is how they show their working and whether they offer the Julian calendar. The comparison below is against the three day of the week calculators that rank highest.
| Feature | Age-Finder | docalculation | 4calculators | allcalci |
|---|---|---|---|---|
| Weekday of any date | Yes | Yes | Yes | Yes |
| Day of the year | Yes | Yes | Yes | Yes |
| ISO week number | Yes | Yes | Yes | Yes |
| Leap year shown | Yes | Not shown | Not shown | Yes |
| Zeller or Doomsday steps shown | Yes, Zeller | Yes | Yes | Yes, Doomsday only |
| Julian calendar option | Yes | Yes | Yes | Not shown |
| Month calendar view | Yes | Yes | Not shown | Not shown |
| This date in nearby years | Yes | Not shown | Not shown | Yes |
| Free, no sign-up or account | Yes | Yes | Yes | Yes |
What is still missing here: the tool shows Zeller’s steps but not the Doomsday rule, which is worked through by hand in the guide. What it adds is the leap year and the days in the month, a Julian calendar tick box, a month calendar view, and the same date in nearby years.
Frequently Asked Questions
What day of the week was I born on?
Enter your date of birth and select Calculate. The Date of Birth Calculator also shows your weekday of birth together with your exact age.
Why does my birthday move to a different weekday each year?
A common year has 365 days, which is 52 weeks and 1 day, so a date moves forward one weekday each year. When a 29 February falls in between, it moves forward two.
What is an ISO week number?
It is the week number defined by ISO 8601: weeks start on Monday, and week 1 is the week containing the year’s first Thursday. It is widely used in business planning and in Europe.
Does every year have 52 weeks?
Every year has 52 full weeks plus one or two days. In ISO numbering, some years therefore have a week 53. 2026 is one of them.
Which weekday does the 13th fall on most often?
Friday. Over the full 400-year Gregorian cycle, the 13th of the month falls on a Friday 688 times, slightly more often than on any other weekday.
How do I plan an event for a particular weekday?
Enter the date you have in mind to check its weekday. If it is not the day you want, the weekday moves by one for each day you shift the date, so a Friday date becomes a Saturday one day later.
The Date Calculator can move a date by a set number of weeks while keeping the same weekday.
Can I check dates in the far past or future?
Yes, any date from the year 1000 to 9999. Dates before 1582 use the Gregorian calendar extended backwards, which can differ from historical records kept in the Julian calendar.
Sources
- ISO 8601: Date and time format (week numbering and the week-year)
- The Gregorian leap-year rule on this page follows the U.S. Naval Observatory leap years page.
- Age-Finder methodology (how exact ages and day counts are calculated).