Day of the Week Calculator - Find the Weekday for Any Date

Determine what day of the week any date falls on with our comprehensive day of the week calculator. Using Zeller's congruence algorithm, instantly discover the exact weekday for any date in the Gregorian calendar. Perfect for historical research, event planning, birthday discovery, and verification. Get instant results with interesting facts about the calculated day.

Day of the Week Calculator

Enter a Date

Select the date you want to find the day of the week for. The calculator works for any date in the Gregorian calendar.

Date

Day of Year

Days Remaining

Occurrence in Month

Week Number (ISO)

Calendar of November 2025

About Tuesday

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What is a Day of the Week Calculator?

A day of the week calculator is a tool that determines which day of the week (Monday, Tuesday, Wednesday, etc.) any given date falls on. Using mathematical algorithms, these calculators can instantly determine the weekday for historical dates, future dates, or any date in between without requiring a physical calendar. The most common algorithm used is Zeller's congruence, developed by Christian Zeller in the 19th century.

Day of the week calculators are useful for numerous applications including verifying birthdays, planning historical research, determining what day major events occurred, and event scheduling. The calculator handles all calendar complexities including leap years and century adjustments automatically.

How to Use the Day of the Week Calculator

Step-by-Step Instructions

Select the Month: Choose the month from the dropdown menu (January through December).
Select the Day: Click the day dropdown and select the day of the month (1-31).
Enter the Year: Type the year into the year field. The calculator works for years from 1582 (when the Gregorian calendar began) through 4902.
Click Calculate: Press the "Calculate" button to determine the day of the week.
Review Results: The calculator instantly displays the day of the week, along with additional information including day of year, days remaining in the year, week number, and interesting trivia about that day.
View Calendar: See the full month calendar with the selected date highlighted for visual reference.
Learn Trivia: Read interesting facts and statistics about the calculated day of the week.
Quick Functions: Use "Today" button for instant calculation of today's day of week, or "Reset" to clear and start over.

Understanding Day of Week Calculations

Zeller's Congruence Formula

Zeller's Congruence (Gregorian Calendar):

h = (q + ⌊(13(m+1))/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7

Where:

h = day of week (0 = Saturday, 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday, 6 = Friday)

q = day of the month

m = month (3 = March, 4 = April, ..., 14 = February)

K = year of the century (year mod 100)

J = zero-based century (⌊year / 100⌋)

⌊ ⌋ = floor function (round down to nearest integer)

Important Notes:

January and February are counted as months 13 and 14 of the previous year. For example, January 2025 is treated as month 13 of 2024.

Example Calculation for November 4, 2025:

q = 4, m = 11, K = 25, J = 20

h = (4 + ⌊(13×12)/5⌋ + 25 + ⌊25/4⌋ + ⌊20/4⌋ − 2×20) mod 7

h = (4 + 31 + 25 + 6 + 5 − 40) mod 7 = 31 mod 7 = 3 (Tuesday)

Day of Week Numbering Systems

ISO 8601 Standard

The ISO 8601 international standard defines Monday as day 1 and Sunday as day 7 of the week. This system is used in many countries and business contexts. The ISO week also defines week numbering that begins with the first week containing a Thursday, which differs from the common Sunday-start week system used in some countries.

US and Traditional Numbering

In the United States and many other countries, Sunday is traditionally considered the first day of the week, numbered 1, with Saturday as day 7. This system is commonly used in calendar displays and business communications.

Monday-First Numbering

Many European and international contexts use Monday as day 1, with Sunday as day 7. This aligns more closely with the ISO standard and reflects the traditional work week structure.

Understanding Zeller's Congruence

Historical Background

Christian Zeller developed his congruence algorithm in 1887. It uses modular arithmetic to calculate the day of the week for any date in the Julian or Gregorian calendar. The algorithm is elegant because it doesn't require knowledge of the number of days in each month or whether it's a leap year—the formula handles all these complexities automatically.

How the Algorithm Works

Zeller's congruence breaks the calculation into components: a base day for the year, adjustments for leap years, adjustments for the specific month, and the day of the month. The formula combines these using modular arithmetic to produce a single digit representing the day of the week. The algorithm works for any date from the beginning of the Gregorian calendar (October 15, 1582) forward.

Why January and February Are Special

In Zeller's algorithm, January and February are treated as months 13 and 14 of the previous year. This convention exists because it allows the leap day calculation to work elegantly within the formula. By moving the leap day to the end of the year mathematically, the formula handles leap years consistently.

Frequently Asked Questions

What is the accuracy range of this calculator?

This calculator uses Zeller's congruence algorithm and works accurately for any date in the Gregorian calendar from October 15, 1582 (when the Gregorian calendar began) through year 4902. For dates before October 15, 1582, you would need to use the Julian calendar calculator. For dates after 4902, the algorithm is still mathematically valid but practically unnecessary.

Can I use this for dates before 1582?

This calculator specifically implements Zeller's congruence for the Gregorian calendar, which officially began on October 15, 1582. For historical dates before this, different calendar systems were in use depending on location. The Julian calendar was the predecessor to the Gregorian calendar, and the Julian calendar version of Zeller's formula uses slightly different calculations. If you need to calculate days for earlier dates, you would need a Julian calendar calculator.

What does ISO week number mean?

ISO 8601 is the international standard for date and time representation. ISO week numbering defines the first week as the week containing the first Thursday of the year. This means some days in late December may belong to week 1 of the next year, and some days in early January may belong to week 52 or 53 of the previous year. This system is widely used in business and data applications for consistency across countries.

How do I verify the calculator's accuracy?

The calculator implements Zeller's congruence correctly. You can verify accuracy by comparing results with a reliable source like a perpetual calendar, or by checking memorable dates you already know. For example, if you know that Christmas 2024 was a Wednesday, you can enter December 25, 2024 to verify the result. The algorithm is mathematically proven and has been in use for over 130 years.

Why are there different day numbering systems?

Different cultures and business contexts traditionally started their weeks on different days. The ISO 8601 standard uses Monday as the first day to provide international consistency, while the US traditionally uses Sunday as the first day of the week. The calculator shows which specific day resulted from your date query, making the numbering system irrelevant to practical use.

What is the 'day of year' result?

Day of year (also called ordinal day) is the number of days from January 1 through the given date. For example, January 1 is day 1, January 2 is day 2, and December 31 is day 365 (or 366 in leap years). This is useful for some scientific and historical applications where dates are referenced by their ordinal day rather than month and day.

Can this calculator predict the day of the week for future dates?

Yes! Zeller's congruence works equally well for past and future dates. You can calculate what day of the week your birthday falls on next year, what day New Year 2050 will be on, or any future date up to year 4902. The algorithm doesn't differentiate between past and future dates.

What happens on leap year dates?

Zeller's congruence handles leap years automatically. February 29 only appears in leap years (years divisible by 4, except century years unless divisible by 400). The algorithm correctly calculates that February 29, 2024 was a Thursday, and recognizes that February 29, 2025 doesn't exist. You don't need to do anything special—just enter the date and the calculator handles it.

How do I interpret the 'nth Occurrence' result?

The nth occurrence indicates which instance of that weekday appears in the selected month. For example, if you select November 4, 2025 (a Tuesday), and the result shows "1st Tuesday", it means November 4 is the first Tuesday in November 2025. Knowing this is useful for identifying holidays like "the fourth Thursday of November" (Thanksgiving) or "the first Monday of September" (Labor Day in the US).

Can I use this for important historical events?

Absolutely! This is one of the most common uses for a day of week calculator. You can verify what day of the week famous historical events occurred. For example, you can find that the moon landing (July 20, 1969) was a Sunday, or that the Titanic sank (April 15, 1912) on a Monday. This adds context to historical understanding and is useful for research and educational purposes.

Interesting Facts About Each Day of the Week

Monday

Statistically the most likely day for U.S. stock market rises. Monday is the only day of the week that is an anagram for one word: "Dynamo." A 2011 study found average people moan for 34 minutes on Mondays versus 22 minutes on other days. Monday is historically the day most people weigh most themselves, and alternative names include "suicide day" due to higher documented suicide rates.

Tuesday

For many employees, Tuesday is the most productive working day. Most job applications are submitted on Tuesday. Black Tuesday (October 29, 1929) was the start of the Great Depression stock market crash. The name derives from Mars, the Roman god of war.

Wednesday

Known as "Hump Day" in U.S. culture—it's in the middle of the workweek. Ash Wednesday is a significant Christian holy day. Bushfires in Australia on February 7, 1979 (Black Wednesday) caused catastrophic damage and losses.

Thursday

In Christian tradition, Maundy Thursday is the day before Easter commemorating the Last Supper. Thirsty Thursday is a social tradition among students and young professionals. Thursday derives from Thor, the Norse god of thunder.

Friday

When Friday falls on the 13th of a month, it's considered bad luck in Western culture. Black Friday (day after Thanksgiving in the U.S.) is the major retail shopping day. Good Friday is the day Christ was crucified according to Christian tradition.

Saturday

In Israel, Saturday is an official day of rest. Saturday is an official election day in Australia and New Zealand. Traditionally a viable day for hunting vampires according to folklore.

Sunday

The first day of the week in many countries. Recognized as a day of rest and worship in Christian and Islamic traditions. Months beginning on Sunday have Friday the 13th that month. Super Bowl Sunday is one of the largest sporting and cultural events.

Historical Examples of Day Calculations

Famous Birthdays

Albert Einstein was born on March 14, 1879—a Friday. George Washington was born on February 22, 1732—a Monday (in the Julian calendar). Cleopatra was born on January 69 BC—which was a Saturday (in the Julian calendar). Using a day of week calculator for famous birthdays adds interesting context to historical knowledge.

Major Historical Events

The Declaration of Independence (July 4, 1776) was signed on a Thursday. The Titanic sank (April 15, 1912) on a Monday. The moon landing (July 20, 1969) occurred on a Sunday. The fall of the Berlin Wall (November 9, 1989) happened on a Thursday.

Matching Modern Dates to History

Christmas 2000 fell on a Monday. Christmas 2025 will be a Wednesday. New Year 2000 was a Saturday. These calculations help establish patterns and plan celebrations based on weekday occurrence.

Best Practices for Using the Calculator

Verification and Cross-Checking

When using calculated day-of-week information for important purposes, verify the results using multiple methods if possible. Compare results with published calendars, historical records, or other reliable calculators to ensure accuracy. For critical applications, document your calculations for audit trails.

Understanding Context

Remember that the day of the week applies to your local time zone. If researching events at different locations, consider that the same moment in time may be different days in different parts of the world. For modern events with global significance, specify the time zone and location.

Historical Calendar Transitions

When working with historical dates near October 1582, remember that some countries transitioned to the Gregorian calendar at different times. Catholic countries adopted it immediately, while other countries adopted it much later (Britain and its colonies in 1752, Russia in 1918). Be aware of calendar transitions when researching historical dates.

Applications and Uses

Genealogy and Family History

Genealogists use day of week calculators to verify dates in historical records and add detail to family history research. Knowing what day of the week an ancestor was born or married adds personal context to historical narratives.

Event Planning and Scheduling

Event planners use these calculators to determine what days of the week major dates fall on for years in advance, allowing optimal scheduling of events, holidays, and observances.

Historical Research and Writing

Historians and authors use day of week information to add accuracy and detail to historical narratives. Knowing the specific day of the week adds vivid detail to accounts of historical events.

Religious and Cultural Observances

Many religious holidays and cultural observances have specific day-of-week requirements. Knowing how to calculate the day of the week allows accurate determination of when holidays occur.

Conclusion

The day of the week calculator uses mathematical algorithms refined over centuries to instantly determine what day of the week any date falls on. Whether verifying historical facts, planning future events, satisfying curiosity about famous dates, or conducting genealogical research, this calculator provides accurate, instant results based on Zeller's congruence algorithm. The Gregorian calendar system combined with proven mathematical formulas ensures reliable day-of-week determination for any date from 1582 forward, making this tool invaluable for anyone needing to know the day of the week for any date.