How far back does this calculator work?

It uses integer calendar arithmetic rather than the JavaScript Date parser and accepts years 1 through 9999. The result depends on whether Gregorian or Julian rules are selected.

Does it account for the Gregorian calendar reform?

You can choose proleptic Gregorian or Julian rules. The tool does not automatically choose a calendar by country because adoption dates differed; use the calendar recorded by your historical source.

What is Zeller's congruence?

Zeller's congruence is a mathematical algorithm devised by Christian Zeller in 1882 to calculate the day of the week for any date in the Gregorian or Julian calendar using only integer arithmetic.

Why does entering month 1 or 2 shift the year?

In Zeller's algorithm, January and February are treated as months 13 and 14 of the previous year. The tool handles this internally — just enter the real month number.

Can I look up future dates?

Yes. The algorithm works for any valid date, past or future, as long as the year is between 1 and 9999.

Day of Week in History

Enter a date to find its weekday under either proleptic Gregorian or Julian calendar rules, plus nearby years when the same date has that weekday.

Enter a Date

Month

Day

Year

Calendar rules

Historical records may use different calendars depending on place and adoption date.

Famous Dates

Enter a date and click Find Day of Week

Calendar scope, method, and privacy

Proleptic Gregorian mode extends current Gregorian rules to every supported year. Julian mode applies Julian leap-year and weekday rules. This tool does not automatically model country-specific calendar adoption or omitted reform dates.

Calculations use integer calendar arithmetic in your browser. Dates are not sent to an external calculation service or stored by this tool.

References: U.S. Naval Observatory Julian dates and NIST calendar resources.

Copied!

Summary

Enter a date to find its weekday under either proleptic Gregorian or Julian calendar rules, plus nearby years when the same date has that weekday.

How it works

Enter a year, month, and day in the input fields.
Choose proleptic Gregorian or Julian calendar rules.
The tool applies the corresponding Zeller congruence using integer arithmetic.
January and February are internally shifted to months 13–14 of the prior year.
The result is mapped to a weekday name (Sunday through Saturday).
The formatted date, day name, and nearby matching years are displayed instantly.

Use cases

Verify what day of the week a historical event occurred on.
Check the weekday of a birthday or anniversary in any past year.
Confirm the day of a contract, document, or legal date.
Satisfy curiosity about famous dates like the moon landing or D-Day.
Plan future dates by confirming which day a year-out date will be.
Research genealogy dates to understand weekly schedules of the era.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu