Day of the Week Calculator

What day of the week was (or will be) any given date? Useful for historical curiosities and scheduling.

Inputs

Result

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How to use this calculator

  • Enter a date — past, present, or future.
  • Read the day name plus the Zeller's-formula derivation in the steps.

About this calculator

Working out the day of the week for an arbitrary date used to require Zeller's congruence and a bit of mental gymnastics. This calculator gives you the answer immediately and shows the Zeller's steps so you can follow the math by hand for any date in the Gregorian calendar.

Frequently asked

What is Zeller's congruence?+
A formula published by German mathematician Christian Zeller in 1882 that computes the day of the week for any date in the Gregorian calendar using only integer arithmetic. It is the standard mental-math approach to "day of the week" puzzles.
Why are January and February treated as months 13 and 14?+
Zeller's formula is simpler when leap-day handling is shifted to the end of the "year" — so January and February are tagged onto the previous year as months 13 and 14. The calculator does this for you.
Does this work for dates before 1582?+
Mathematically yes, but historically the Gregorian calendar was not in use before 1582. For the Julian calendar (used before that), there is a separate Zeller variant. This calculator uses the Gregorian formula throughout, which matches what most modern date libraries do.
How is "weekend" defined?+
Saturday and Sunday — the most common Western definition. Other regions use Friday/Saturday or different weekend conventions; for those, ignore the weekend flag.
Can I use this to verify my birth day-of-week?+
Absolutely — enter your date of birth and read the result. Many people are surprised to learn they were born on a Tuesday or Thursday.

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