Day of the Week Calculator
Find the weekday of any date from year 1 to 9999, such as the day you were born. See the Doomsday rule worked out, the week number, and what day the date falls on in other years.
Old dates were often written in the Julian calendar.
Today, unless you change it.
How your weekday is worked out
The Doomsday rule: a mental-maths trick you can do in your head, checked against the computer's calendar.
- 1
Century anchor
1900s anchor day
Wednesday - 2
The year's doomsday
90 → 7+6+1 = 14
Wed + 14 → Wed - 3
The month's doomsday
May → 9 May
9 May 1990 = Wed - 4
Count to your day
15 − 9 = +6 days
Tuesday
Read more: the formula, Zeller's congruence, a worked example and the 1582 and 1752 calendar changes
Every weekday answer here comes from counting days with the Julian Day Number, a running day count used by astronomers. The Doomsday rule above is the same answer done as mental maths, and the calculator checks both against Zeller's congruence and your device's own calendar.
The formula
Doomsday rule:
1. century anchor: 1800s Friday, 1900s Wednesday, 2000s Tuesday, 2100s Sunday (repeats every 400 years)
2. take the last two digits of the year: a = how many 12s, b = what is left, c = how many 4s in b
3. the year's doomsday = anchor + a + b + c, minus whole weeks
4. weekday = the year's doomsday + (your day − that month's doomsday date), minus whole weeks
Zeller's congruence (Gregorian):
h = (day + ⌊13 × (month + 1) ÷ 5⌋ + K + ⌊K ÷ 4⌋ + ⌊J ÷ 4⌋ + 5 × J) mod 7
K = last two digits of the year, J = the century; January and February count as months 13 and 14 of the year before
h: 0 Saturday, 1 Sunday, 2 Monday, 3 Tuesday, 4 Wednesday, 5 Thursday, 6 Friday
Julian calendar: use 5 + 6 × J in place of ⌊J ÷ 4⌋ + 5 × J
Worked example
What day was 15 May 1990? With the Doomsday rule: the 1900s anchor is Wednesday. The year ends in 90, which is 7 twelves (a = 7) with 6 left over (b = 6), and 6 holds one 4 (c = 1). 7 + 6 + 1 = 14, which is exactly 2 weeks, so the 1990 doomsday is also a Wednesday. In May the doomsday date is 9 May, so 9 May 1990 was a Wednesday. 15 is 6 days after 9, and Wednesday + 6 days is Tuesday.
With Zeller's congruence: day = 15, month = 5, K = 90, J = 19. ⌊13 × 6 ÷ 5⌋ = ⌊15.6⌋ = 15, ⌊90 ÷ 4⌋ = 22, ⌊19 ÷ 4⌋ = 4 and 5 × 19 = 95. 15 + 15 + 90 + 22 + 4 + 95 = 241, and 241 = 34 × 7 + 3, so h = 3: Tuesday again.
15 May 1990 is day 135 of 365 (31 + 28 + 31 + 30 + 15), in ISO week 20, and it was the 3rd Tuesday of May. In the Julian calendar the same day was 2 May 1990, 13 days earlier.
Doomsday dates for each month
| Month | Doomsday date | Memory aid |
|---|---|---|
| January | 3 (4 in a leap year) | The 3rd, 3 years in 4 |
| February | 28 (29 in a leap year) | The last day of February |
| March | 14 | Pi day, 3/14 |
| April | 4 | 4/4 |
| May | 9 | "9 to 5": 9 May and 5 September |
| June | 6 | 6/6 |
| July | 11 | "7-Eleven": 11 July and 7 November |
| August | 8 | 8/8 |
| September | 5 | "9 to 5": 9 May and 5 September |
| October | 10 | 10/10 |
| November | 7 | "7-Eleven": 11 July and 7 November |
| December | 12 | 12/12 |
The 1582 and 1752 calendar changes
The Julian calendar adds a leap day every 4 years, which makes its year about 11 minutes too long. By 1582 it had drifted 10 days, so Pope Gregory XIII dropped 5 to 14 October 1582 and stopped leap years in century years not divisible by 400 (1700, 1800 and 1900 were not leap years, 2000 was). Spain, Portugal, Italy and Poland switched at once; Britain and its colonies waited until 1752, when Wednesday 2 September was followed by Thursday 14 September. Russia switched in 1918 and Greece in 1923. The weekday cycle never broke at any switch.
By default this calculator uses the Gregorian calendar for every year ("proleptic" Gregorian), which is what most software does. For a date written in an old record, pick the calendar the record used in Advanced. Before 1752 England also started its legal year on 25 March, so a date like 1 February 1700 in an English record may mean 1701 in today's numbering.
Monday's child
The fun line under your answer comes from the traditional English rhyme "Monday's Child", first printed in A. E. Bray's Traditions of Devonshire (1838). It is folklore, not a forecast.
Common questions
What day of the week was I born?
Enter your date of birth and the calculator shows the weekday straight away, with the Doomsday rule working step by step. For example, 15 May 1990 was a Tuesday, and so was 29 February 2000.
How does the Doomsday rule work?
In any year, 4/4, 6/6, 8/8, 10/10, 12/12, 9 May, 5 September, 11 July, 7 November, 14 March and the last day of February all fall on the same weekday, called the doomsday. Work out the year's doomsday from the century anchor and the last two digits of the year, then count from the nearest of those dates to yours.
Why can't I enter 10 October 1582?
With the "Switch in 1582" calendar, 5 to 14 October 1582 never happened: Pope Gregory XIII's reform jumped from Thursday 4 October straight to Friday 15 October. Britain and its colonies made the same change in 1752, going from 2 September to 14 September. Choose Gregorian or Julian in Advanced to work with those dates anyway.
How many Friday the 13ths are there in a year?
Every year has at least one and at most three. 2026 has three: February, March and November. A month has a Friday the 13th whenever it starts on a Sunday.
When will my birthday fall on the same weekday again?
A date moves on one weekday each year, or two when a 29 February comes in between, so the same weekday comes back after 5, 6 or 11 years. The whole Gregorian calendar repeats exactly every 400 years, because 400 years is 146,097 days, which is exactly 20,871 weeks.
What is the ISO week number?
ISO 8601 weeks start on Monday, and week 1 is the week that has the year's first Thursday in it. So 1 January 2100, a Friday, belongs to week 53 of 2099.
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Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.