What is the Doomsday rule for mental day-of-the-week calculation?

Created by mathematician John Horton Conway, the Doomsday rule assigns a specific 'anchor' day to each year called the Doomsday. Certain memorable dates in every year always fall on this exact same Doomsday: 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February.

Why did 11 days go missing in September 1752?

When Britain and its American colonies adopted the Gregorian calendar to replace the Julian calendar in September 1752, Wednesday September 2 was followed immediately by Thursday September 14 to correct 11 days of accumulated solar drift.

Is Sunday or Monday officially the first day of the week?

According to the international standard ISO-8601, Monday is officially designated as day 1 of the week. However, in the United States, Canada, and traditional religious calendars, Sunday is placed as the first day.

How often do calendar day-and-date combinations repeat?

Due to the 4-year leap year cycle and 7-day week, calendars repeat every 28 years (4 × 7) within a single century, with interim partial repeats occurring every 6 or 11 years.

Can I use this calculator to find the day I was born?

Yes! Simply choose your birthday in the date picker. The calculator will tell you the exact day of the week you entered the world, along with your birth year's leap year status and day rank.

Everyday

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Day of the Week Calculator — Find the Day for Any Past or Future Date

Free online Day of the Week Calculator. Discover what day of the week you were born on, check historical event weekdays, find leap years, and compute ISO week numbers for any date.

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How to use this calculator

👉 Fill in the boxes below and your answer appears instantly — no maths needed, we do it all for you! 🎉

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In plain English — what does this do?

📊 A helper for daily life! Things like working out your age, how much to tip, what a sale price is, or how long until your birthday. Quick and easy answers for real life!

📌 Direct Answer & Summary

A Day of the Week Calculator finds the exact weekday (Monday through Sunday) for any historical, current, or future date in the Gregorian calendar. Using mathematical algorithms like Zeller's Congruence or Sakamoto's method, it determines calendar days, ISO week numbers, day-of-the-year rank, and leap year cycles.

📅 Select Any Date

Famous Date Presets:

Day of the Week

Sunday

July 20, 1969

Day of Year

Day 201

ISO Week Number

Week 29

Leap Year?

No (365 Days)

Day Category

Weekend

What is Day of the Week Calculator — Find the Day for Any Past or Future Date?

How to use it

1️⃣ Select or type your target Date using the calendar picker, or choose a famous historical date preset.
2️⃣ Instantly see the exact day of the week prominently displayed.
3️⃣ Review supplementary calendar details including whether the year is a Leap Year, the ISO-8601 week number, and the cumulative day number (1–366).
4️⃣ Check how many days remain until this exact calendar anniversary recurs.

Formula & Variables

Zeller's Congruence Algorithm: h = [q + ⌊13(m + 1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J] mod 7 Where q = day of month, m = month (March=3...Jan=13 of prev year), K = year mod 100, and J = ⌊year/100⌋.

Variable Definitions:

h Day of the week index (0 = Saturday, 1 = Sunday ... 6 = Friday)

q Day of the month (1 through 31)

m Adjusted month number (January and February counted as months 13 and 14 of previous year)

K Year of the century (Year mod 100)

J Zero-based century (Floor of Year ÷ 100)

💡 See it in action — a real example

What day was July 20, 1969 (Apollo 11 Moon Landing)? Applying Zeller's formula yields h = 1, which corresponds to Sunday. Neil Armstrong stepped onto the lunar surface on a Sunday evening.

📊 Day of the Week Names Across Major World Languages

Cross-linguistic comparison of planetary and numeric weekday names.

English	Spanish	French	German	Hindi	Japanese
Monday	Lunes	Lundi	Montag	Somvar (सोमवार)	Getsuyōbi (月曜日)
Tuesday	Martes	Mardi	Dienstag	Mangalvar (मंगलवार)	Kayōbi (火曜日)
Wednesday	Miércoles	Mercredi	Mittwoch	Budhvar (बुधवार)	Suiyōbi (水曜日)
Thursday	Jueves	Jeudi	Donnerstag	Guruvar (गुरुवार)	Mokuyōbi (木曜日)
Friday	Viernes	Vendredi	Freitag	Shukravar (शुक्रवार)	Kinyōbi (金曜日)
Saturday	Sábado	Samedi	Samstag	Shanivar (शनिवार)	Doyōbi (土曜日)
Sunday	Domingo	Dimanche	Sonntag	Ravivar (रविवार)	Nichiyōbi (日曜日)

* Most romance and Sanskrit languages name weekdays after the classical seven celestial planets.

❓ Common questions

What is the Doomsday rule for mental day-of-the-week calculation?: Created by mathematician John Horton Conway, the Doomsday rule assigns a specific 'anchor' day to each year called the Doomsday. Certain memorable dates in every year always fall on this exact same Doomsday: 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February.
Why did 11 days go missing in September 1752?: When Britain and its American colonies adopted the Gregorian calendar to replace the Julian calendar in September 1752, Wednesday September 2 was followed immediately by Thursday September 14 to correct 11 days of accumulated solar drift.
Is Sunday or Monday officially the first day of the week?: According to the international standard ISO-8601, Monday is officially designated as day 1 of the week. However, in the United States, Canada, and traditional religious calendars, Sunday is placed as the first day.
How often do calendar day-and-date combinations repeat?: Due to the 4-year leap year cycle and 7-day week, calendars repeat every 28 years (4 × 7) within a single century, with interim partial repeats occurring every 6 or 11 years.
Can I use this calculator to find the day I was born?: Yes! Simply choose your birthday in the date picker. The calculator will tell you the exact day of the week you entered the world, along with your birth year's leap year status and day rank.