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Day of the Week Calculator

Instantly find the day of the week for any date in history or the future — simply enter a date and get the result in seconds.

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Friday
June 12, 2026
Weekday

How it works

This calculator uses the proleptic Gregorian calendar. Enter any date from year 1 to 9999 — including birthdays, holidays, and historical events — and the result appears instantly in your browser without sending data to any server.

Quick answer

To find what day of the week a date falls on, use Tomohiko Sakamoto's algorithm or Zeller's congruence, both of which work for any Gregorian calendar date. For example, January 1, 2000 was a Saturday, July 4, 1776 was a Thursday, and December 25, 2025 is a Thursday. You can also count days from a known reference point: January 1, 2026 is a Thursday, so adding days and taking modulo 7 gives the weekday offset. This calculator handles all dates from the year 1 through 9999 in the proleptic Gregorian calendar.

Formula & method

h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7
  • h — Day of week (0=Saturday, 1=Sunday, 2=Monday, …, 6=Friday)
  • q — Day of the month
  • m — Month (3=March … 14=February)
  • K — Year of the century (year mod 100)
  • J — Zero-based century (⌊year/100⌋)

Zeller's congruence for the Gregorian calendar. January and February are treated as months 13 and 14 of the previous year.

Examples

Example 1: January 1, 2000
Input
Date: January 1, 2000
Result
Saturday
Why
January 1, 2000 was a Saturday — the dawn of the new millennium. Using the reference that January 1, 1900 was a Monday, and accounting for 100 years of leap-year cycles (24 leap years from 1900–1999 inclusive), the day advances by (365×100 + 24) mod 7 = (36500 + 24) mod 7 = 36524 mod 7 = 6 days, landing on Saturday.
Example 2: July 4, 1776 (US Independence Day)
Input
Date: July 4, 1776
Result
Thursday
Why
The United States Declaration of Independence was signed on a Thursday. Historical records from delegates' diaries and congressional journals confirm July 4, 1776 fell on a Thursday, a well-documented historical fact.
Example 3: December 25, 2025 (Christmas)
Input
Date: December 25, 2025
Result
Thursday
Why
January 1, 2025 is a Wednesday. Counting days from January 1 to December 25: 31+28+31+30+31+30+31+31+30+31+30+25 = 359 days total, meaning December 25 is day 359. Day offset from Jan 1 = 359 − 1 = 358 days. 358 mod 7 = 1. Wednesday + 1 = Thursday. Christmas 2025 falls on Thursday.
Example 4: June 11, 2026
Input
Date: June 11, 2026
Result
Thursday
Why
January 1, 2026 is a Thursday. Days from January 1 to June 11: 31(Jan)+28(Feb)+31(Mar)+30(Apr)+31(May)+11(Jun) = 162 days total; June 11 is day 162, so offset from Jan 1 = 162 − 1 = 161 days. 161 mod 7 = 0. Thursday + 0 = Thursday. June 11, 2026 is a Thursday.

Frequently asked questions

What day of the week was I born on?

Enter your birth date into the calculator and it will instantly tell you the day of the week you were born. For example, someone born on January 1, 2000 was born on a Saturday.

What algorithm does this calculator use?

This calculator uses JavaScript's built-in Date object, which implements the proleptic Gregorian calendar. Internally, JavaScript computes the day of the week using epoch-based arithmetic (milliseconds since January 1, 1970), which is mathematically equivalent to Zeller's congruence or Tomohiko Sakamoto's algorithm.

Does the calculator work for dates before 1970?

Yes. JavaScript's Date object supports dates well before 1970. The calculator works for any date from year 1 through 9999, including historical dates like July 4, 1776 (Thursday) and January 1, 1900 (Monday).

What is the difference between the Gregorian and Julian calendar?

The Gregorian calendar, introduced in 1582, corrected the Julian calendar's drift by adjusting leap year rules. Before 1582, many countries used the Julian calendar. This calculator uses the proleptic Gregorian calendar for all dates, meaning it applies Gregorian rules even to dates before 1582.

What day of the week does January 1, 2027 fall on?

January 1, 2027 falls on a Friday. Since January 1, 2026 is a Thursday and 2026 is not a leap year, January 1, 2027 is exactly 365 days later. 365 mod 7 = 1, so Thursday + 1 = Friday.

How do I find what day a future holiday falls on?

Simply enter the date of the holiday into the calculator. For recurring holidays like Christmas (December 25) or New Year's Day (January 1), just change the year to the one you want. The calculator instantly shows the weekday for any year you choose.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

  • ✓ Free to use
  • ✓ No sign-up required
  • ✓ Runs entirely in your browser — nothing is uploaded.
  • ✓ Formula and method shown above
Last updated: How we calculateReport a correctionDisclaimer

Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.

Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.

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