Free day of the week calculator
Find out what day of the week any date falls on — past, present, or future. Enter a birthday, a deadline, or a historical date and this calculator names the weekday using the same date math your device's calendar uses, updated live, as you type.
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Date calculations use calendar days. Working days exclude weekends but not public holidays unless noted.
Results are estimates. Consult a professional.
What is a day of the week calculator?
A day of the week calculator tells you which of the seven weekdays a given calendar date falls on — Sunday through Saturday. Enter any date, past or future, and it answers instantly: what day was I born, what day is my birthday this year, what weekday a deadline lands on. This tool computes the result the moment you change the date, with no lookup table needed.
Working out a weekday by hand is surprisingly fiddly because the calendar repeats on a 7-day cycle that is knocked out of step by leap years. A calculator removes the arithmetic: it applies the same logic your computer's calendar uses and returns the named day directly.
How to find what day of the week a date is
Using the calculator takes one step — type or pick a date and read the result. Here is what each part means:
- Enter the date you care about — your birthday, a historical event, or a future deadline. The field accepts a full year, month, and day.
- Read the weekday shown as the headline result (for example, "Monday").
- Confirm the sentence below it, which restates the date and its day so there is no ambiguity — e.g. "2026-06-01 is a Monday."
How the day of the week is calculated
Behind the scenes the calculator does what your device's calendar does: it counts the date as a position in time and takes that position modulo 7 to land on one of the seven weekdays. Mathematicians have two classic hand-methods that reach the same answer — Zeller's congruence and Conway's Doomsday rule — and they are worth knowing because they show why leap years complicate the count.
Zeller's congruence
Zeller's congruence, published by Christian Zeller in the 1880s, is a single modular-arithmetic formula. It numbers months from March (3) through February (14), treating January and February as months 13 and 14 of the previous year so the leap day always sits at the end of the count.
Conway's Doomsday rule
John Conway's Doomsday rule (1973) is a mental shortcut. Within any year, a set of easy dates — 4/4, 6/6, 8/8, 10/10, 12/12, plus the last day of February — all share the same weekday, the year's "doomsday." Find the doomsday, then count forward or back to your target date. For the 1900s the century anchor is Wednesday; for the 2000s it is Tuesday.
This calculator does not need either trick — it relies on the date arithmetic built into the browser — but both methods return the identical weekday, and they explain why a date's weekday creeps forward one day most years and two days across a leap day.
A worked example: what day is 1 June 2026?
Sam is scheduling a launch for 1 June 2026 and wants to know the weekday before sending invites. Here is how the calculator — and Zeller's congruence — reach the same answer.
Step 1 — Set up the values for Zeller's congruence
June is month 6, so no January/February adjustment is needed. q = 1 (the day), m = 6, the year stays 2026, so K = 26 (year within the century) and J = 20 (the century).
Step 2 — Plug the numbers into the formula
Step 3 — Read the result
Both the formula and the calculator agree: 1 June 2026 is a Monday. The widget shows "Monday" as the headline and confirms it with "2026-06-01 is a Monday."
What day of the week was I born?
The most popular use of this tool is answering "what day of the week was I born?" Enter your date of birth and the calculator names the weekday. There is a one-in-seven chance for each day, but the spread is not perfectly even across a lifetime because leap years nudge the pattern.
Type 1990-07-15 and the calculator returns Sunday — that birthday fell on a Sunday. Because each common year shifts a fixed date forward by one weekday (and two across a leap day), the same 15 July landed on a Monday in 1991 and a Tuesday in 1992 before a leap-year jump.
To see how your birthday weekday changes year to year, or to count the exact days you have been alive, pair this with the age calculator and the days between dates calculator.
Doomsday dates and century anchors
These reference points make the Doomsday rule quick. Every date in the first table shares one weekday within a given year (the year's doomsday), and the second table gives the anchor weekday for each century.
| Memorable doomsday date | Written as | Notes |
|---|---|---|
| 4 April | 4/4 | Even month, day matches month |
| 6 June | 6/6 | Even month, day matches month |
| 8 August | 8/8 | Even month, day matches month |
| 10 October | 10/10 | Even month, day matches month |
| 12 December | 12/12 | Even month, day matches month |
| Last day of February | 28 or 29 Feb | 29th in a leap year, 28th otherwise |
| 9 May / 5 Sept | 5/9 and 9/5 | Odd-month pair ("9-to-5") |
| 11 July / 7 Nov | 7/11 and 11/7 | Odd-month pair ("7-Eleven") |
All of these dates fall on the same weekday within any single year, so finding one tells you the rest.
| Century | Doomsday anchor weekday |
|---|---|
| 1800s (1800–1899) | Friday |
| 1900s (1900–1999) | Wednesday |
| 2000s (2000–2099) | Tuesday |
| 2100s (2100–2199) | Sunday |
The century anchor is the starting weekday before the year-within-century adjustment; the anchors repeat on a 400-year cycle.
Common day of the week questions
How accurate is a day of the week calculator?
For any Gregorian date it is exact. The weekday is fixed mathematics, not an estimate — Zeller's congruence, the Doomsday rule, and your device's calendar all return the same named day for a given date.
Why does my birthday fall on a different weekday each year?
Because 365 is not a multiple of 7. A common year is 52 weeks plus one day, so a fixed date moves forward one weekday each year — and two weekdays when a 29 February falls in between. That is why your birthday weekday usually advances by one and occasionally skips by two.
Does the calculator handle dates far in the past or future?
Yes, for Gregorian calendar dates. Note that countries adopted the Gregorian calendar at different times (most from 1582 onward), so for dates before a country's switch the historical local weekday may differ from the Gregorian result.
How this calculator works and sources
This day of the week calculator reads the date you enter and returns the weekday using the browser's built-in date arithmetic, which counts the day's position and takes it modulo 7 — the same result Zeller's congruence and Conway's Doomsday rule produce by hand. Everything runs locally in your browser; no date is sent anywhere. The formulas, doomsday anchors, and weekday-shift facts below are standard, well-documented calendar mathematics.
Wikipedia — Zeller's congruence (the Gregorian weekday formula and month-numbering convention).Wikipedia — Doomsday rule (Conway's mental algorithm, doomsday dates, and century anchors).timeanddate.com — Weekday Calculator: what day a date falls on.Frequently asked questions about the free day of the week calculator
About this day of the week calculator
This day of the week calculator runs entirely in your browser. It reads the date you enter and returns the weekday — Sunday through Saturday — using built-in date arithmetic that matches Zeller's congruence and the Doomsday rule. No date is sent anywhere, there is no submit button, and the answer changes the instant you edit the date.
Looking for related date tools? Browse the full everyday calculators shelf, or see every tool in the calculators directory. To go further with dates, try the age calculator or the days between dates calculator.