Day of the Week Calculator

Spin the Dial

Famous Dates

Tap a historic date to see its weekday on the dial.

Year-Doomsday Reference

Curious whether a year has a Feb 29? Check the leap-year calculator.

The Doomsday Algorithm

Century Anchor Days

No saved lookups yet. Tap "Save to history" to remember up to eight dates.

How to Use the Dial

1. Pick any date with the date picker — the dial pointer rotates to land on the correct weekday segment immediately.
2. Read the big weekday name under the dial, and the hero badges for the year's Doomsday and whether it is a leap year.
3. Follow the five Doomsday steps in the side panel — century anchor, year adjustment, month anchor, and the final day count — to see how the weekday is derived.
4. Tap a famous date preset (Moon landing, Y2K, your country's independence) to see its weekday, or press Today.
5. Save the lookup to your browser history and revisit it later by tapping the saved card.

A Brief History of Weekday Math

In 2026, a genealogist verifying a birth record, a trivia host preparing a pub quiz, and an adult who simply wants to know what day of the week they were born all reach for the same answer: the weekday of an arbitrary date. This tool spins a perpetual-calendar dial to the right weekday for any date from the 1700s to the 2100s, then shows the Doomsday algorithm steps that prove the result rather than just asserting it.

The mathematics of weekdays is older than computers. The modern shortcut shown here is the Doomsday algorithm, devised by the British mathematician John Horton Conway in 1973. Conway — better known for the Game of Life cellular automaton — noticed that within any given year a set of easy-to-remember dates all fall on the same weekday, which he called the year's Doomsday. Examples include 4/4, 6/6, 8/8, 10/10, and 12/12, plus the last day of February. Anchor those and you can compute any date in your head in seconds.

Each century has its own anchor day. For the Gregorian calendar the anchor cycles through Tuesday, Sunday, Friday, and Wednesday for the 1700s, 1800s, 1900s, and 2000s respectively, then repeats every 400 years because the Gregorian leap rule itself repeats on a 400-year cycle. The 1900s anchor is Wednesday, which is why so many dates in living memory are easy to pin: anyone working from a 20th-century birth year starts from Wednesday.

The full method is compact. Take the last two digits of the year, call them YY. Divide YY by 12 and keep the whole number; add the remainder; add the whole number of that remainder divided by 4; then add the century anchor. Take the result modulo 7 and that is the year's Doomsday weekday. From there, count days forward or back from the nearest Doomsday date in the target month. The dial in this tool performs exactly these steps and animates to the answer.

The Gregorian calendar that the algorithm assumes was introduced by Pope Gregory XIII in 1582 to correct the drift of the older Julian calendar. Different countries adopted it at different times — Catholic Europe in 1582, Britain and its colonies in 1752 (when 11 days were dropped), and Russia not until 1918 — so for dates before a country's adoption the historical weekday may differ from the proleptic Gregorian weekday this tool computes. For all dates from the mid-1700s onward in the English-speaking world, the result matches the historical record.

Before Conway, the standard formula was Zeller's congruence, published by the German mathematician Christian Zeller in 1882 and 1883. Zeller's congruence is a single modular-arithmetic expression that any computer evaluates instantly, and it is the method most date libraries actually use internally. The Doomsday algorithm is favoured for mental calculation and teaching because its steps are intuitive; this tool cross-checks the dial against the native date routine so the displayed weekday is always correct regardless of method.

Knowing the weekday of a date matters more than it first appears. Payroll and billing systems must know whether a deadline falls on a weekend; historians date events and letters; software must validate that, say, 29 February 2024 really was a Thursday; and people simply enjoy the small delight of learning their birth weekday. Paired with the leap-year checker and the day-of-year counter, this perpetual calendar turns any date into a fully decoded position in the Gregorian system.