Day of Week Calculator
Determine the day of the week for any date in history or the future. Switch between Julian and Gregorian calendar systems, view a visual calendar of the selected month, translate day names across languages, and check which years share the exact same weekday alignment.
Enter any date to find the day of the week. Supports Gregorian and Julian calendars with automatic detection at the 1582 calendar reform boundary.
Why Use Our Day of Week Calculator?
Dual Calendar Support with Auto-Detection
Automatically applies Gregorian calendar rules for dates after October 14, 1582 and Julian calendar rules for earlier dates - matching the historical record. You can also manually lock to either system.
Visual Monthly Calendar Grid
Displays an interactive monthly grid for the selected date, highlighting the target day. Instantly see which day of the month falls on which weekday for the entire month.
Weekday Translations in 6 Languages
Shows the day name translated into Spanish, French, German, Italian, Portuguese, and Japanese alongside the English name. Useful for international date references.
Fully Private - All Calculations in Browser
All date calculations run locally using JavaScript. No dates or queries are sent to any server. Complete privacy with no signup, no tracking, and no data collection.
Common Use Cases for Day of Week Calculator
Finding Your Birth Day of the Week
Discover what day of the week you were born on. Enter your birth date and instantly see the weekday along with the nursery rhyme character associated with that day.
Historical Event Research
Find the weekday of any historical event - battles, treaties, elections, or births. The dual calendar system handles pre-1582 dates correctly using Julian calendar rules, matching historians' records.
Planning Future Dates
Check what day of the week a future date falls on before scheduling events, deadlines, or appointments. Confirm whether a specific date is a weekday or weekend without opening a calendar app.
Genealogy & Family History
Genealogists use the day of week calculator to verify dates in old documents. Marriage certificates, birth records, and church logs often specify the weekday, which can confirm or flag transcription errors.
Astronomy & Calendar Studies
Astronomers and calendar researchers use this tool to cross-reference dates across the Julian-Gregorian calendar transition and verify computed dates against historical astronomical records.
Finding Repeating Calendar Years
See which years share the same weekday alignment for a given date. Useful for long-term event planning, recurring anniversaries, or academic calendar research.
How the Day of Week Calculator Works
Zeller's Congruence Algorithm
This calculator uses Zeller's Congruence, a mathematical formula developed by Christian Zeller in 1882 that computes the day of the week for any calendar date using modulo arithmetic on the year, month, and day values. It is the same algorithm used in many programming languages for date calculations. The formula handles all calendar leap-year rules and month-length variations internally.
The Julian-Gregorian Calendar Transition (1582)
The Julian calendar - introduced by Julius Caesar in 45 BC - drifts slightly relative to the solar year because it defines a leap year as any year divisible by 4. By 1582 this accumulated error had reached 10 days. Pope Gregory XIII reformed the calendar, decreeing that October 4, 1582, would be followed immediately by October 15, 1582. The days October 5-14, 1582 simply never occurred. The auto-detect mode in this calculator applies Julian rules for dates on or before October 4, 1582 and Gregorian rules for dates on or after October 15, 1582 - matching the standard historical convention used by astronomers and historians.
Julian vs Gregorian Leap Year Rules
- Julian: Any year divisible by 4 is a leap year - simple but accumulates error over centuries (11 minutes per year).
- Gregorian: A year is a leap year if divisible by 4, unless also divisible by 100, in which case it must also be divisible by 400. So 1900 was not a leap year but 2000 was. This keeps the calendar aligned within 26 seconds per year.
Same-Weekday Year Lookup
The calculator searches a ±100 year window around the input year to find other years where the same date falls on the same weekday. Calendar years repeat their weekday pattern roughly every 6, 11, or 28 years depending on intervening leap years. This feature is useful for finding equivalent calendar years for planning purposes.
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Frequently Asked Questions About Day of Week Calculator
A Day of Week Calculator computes the exact weekday (Sunday through Saturday) for any given date. It is commonly used to find birth weekdays, look up historical event days, or confirm what day a future date falls on.
This tool uses Zeller's Congruence, a mathematical formula developed by Christian Zeller in 1882. It performs modulo arithmetic on the year, month, and day values. All calculations run locally in the browser - no date queries are sent to a server.
The Julian calendar drifts from the solar year by about 11 minutes annually. By 1582 this had accumulated to 10 days of error. Pope Gregory XIII reformed the calendar, ordering that October 4, 1582 be followed by October 15, 1582. The days October 5-14, 1582 never existed.
Yes. The auto-detect mode applies Julian calendar rules for dates on or before October 4, 1582 and Gregorian rules for dates on or after October 15, 1582 - matching the standard historical convention. You can also manually lock to either calendar system.
Julian: any year divisible by 4 is a leap year. Gregorian: a year is a leap year if divisible by 4, unless divisible by 100, in which case it must also be divisible by 400. So 1900 was not a Gregorian leap year but 2000 was.
It shows other years (within ±100 years of your input) where the same month and day also falls on the same weekday. Calendar patterns repeat roughly every 6, 11, or 28 years depending on intervening leap years.
No. All calculations run entirely client-side in your browser using JavaScript. No dates, queries, or IP addresses are sent to any server.
Yes - completely free with no signup, no account, and no usage limits. Calculate the weekday for any date as often as you need.