How any date gets its weekday
A calendar's seven-day week has run in an unbroken cycle for so long that working out the weekday of a distant date looks like it needs a lookup table, when really it only needs one number: how many days sit between that date and one you already know, taken modulo seven. Seven days is one full lap of the week, so any date exactly seven, fourteen or seven hundred days from today falls on the same weekday today does. The only real work is counting the days in between correctly, which means respecting real month lengths and real leap years rather than an averaged year length. This calculator does that count directly from the calendar date you enter, and shows the same answer worked out a second way below: the doomsday trick, the mental shortcut this page is really about.
The doomsday trick, step by step
The doomsday rule was popularised by the mathematician John Conway as a way to work out any weekday in your head, and it rests on one observation: within any single year, a handful of very easy-to-remember dates all fall on the same weekday, called that year's "doomsday". Once you know doomsday for a year, you can reach any other date in that year in a short hop.
Step 2 is the "12s, 4s, 7s" count: take the year's last two digits, see how many 12s fit in, take what is left over, see how many 4s fit into that remainder, then add the three numbers together and take the result modulo 7. Add that to the century's anchor weekday and the result is doomsday for that year. Working example: for 1994, the last two digits are 94. 94 divided by 12 goes 7 times with 10 left over; 10 divided by 4 goes 2 times. 7 plus 10 plus 2 is 19, and 19 modulo 7 is 5. The 1900s anchor is Wednesday, and Wednesday moved on 5 days lands on Monday, so every doomsday date in 1994 is a Monday, 14 March included. Your own date's version of this working, with your own numbers, is shown in the results panel once you pick a date.
The dates worth memorising
Step 3 only works because a short list of dates recur across every month. The four, six, eight, ten and twelfth months each carry a doomsday on the day matching their own number: 4 April, 6 June, 8 August, 10 October, 12 December. The odd months pair up the same way once shifted five, sometimes summarised as "I work 9 to 5 at the 7-Eleven": 5 September, 9 May, 7 November, 11 July. January and February depend on whether the year is a leap year, since the doomsday needs to fall after February's leap day has already been accounted for: 3 January in a common year, 4 January in a leap year, then the last day of February itself, 28 or 29. March breaks the pattern in the nicest way: its doomsday date is 14 March, Pi Day, which is easy to remember for reasons that have nothing to do with calendars at all.
Learning these twelve dates and the century anchors is the entire trick; the arithmetic in step 2 gets quick with practice, since the numbers involved are always small. It will not out-run a calculator, but it turns "what day was that" from a lookup into something worked out on the spot.
Why the weekday shifts by one day a year, two after a leap year
A common year has 365 days, and 365 divided by 7 leaves a remainder of 1, so a fixed date lands one weekday later than it did the year before: a birthday on a Tuesday becomes a Wednesday. A leap year adds a 366th day, a remainder of 2, so if a 29 February falls between one occurrence of a date and the next, that date jumps forward two weekdays instead of one. Because leap years land roughly every fourth year, with an occasional skip at a century that is not divisible by 400, the weekday of a fixed date drifts in a pattern that never quite settles into a clean cycle. That is the whole reason a page like this one is more useful than a mental shortcut alone: the "next 10 years" table below tracks that drift for the exact date entered, leap years and all.
The eleven lost days of 1752
Britain and its colonies ran on the older Julian calendar until 1752, when Wednesday 2 September was, by act of Parliament, followed directly by Thursday 14 September, deleting eleven calendar dates that had simply drifted out of step with the sun. Most of Catholic Europe made the same switch to the Gregorian calendar far earlier, in October 1582, when only ten days needed removing, because the drift was smaller at that point; a scattering of other countries held out for centuries longer still. This calculator answers every date on the modern Gregorian calendar, projected backward, which is standard practice and matches what every other date tool does, but it means a date from before your country's own switch shows the weekday the modern calendar assigns it, not necessarily the one written on a document at the time.
Reading your results
Alongside the weekday itself, the panel above shows which numbered day of the year your date falls on, useful for anything counted in day-of-year terms rather than a calendar date. The band shows the same thing visually, how far through its year the date sits. The birthday-planning table lists the same date across the next ten years so a recurring event, a birthday party, an anniversary, a renewal date, can be checked against the weekday it will land on well ahead of time; a 29 February is shown clamped to 28 February in the years that do not have one, noted plainly rather than silently. The bar chart totals up how often each weekday comes up across those ten years, which makes the irregular drift from the section above visible rather than abstract. For working out how many days sit between two dates rather than a single weekday, the Date Calculator does that; for turning a date of birth into an exact age, see the Age Calculator.
Questions people ask
How far back and forward does this calculator work?
From the year 1600 to the year 2400. Every date in that stretch is worked out on the modern Gregorian calendar, which is the calendar almost every date tool, spreadsheet and computer clock uses by default.
Why not go back further than 1600?
Before the Gregorian calendar existed at all, and for a long stretch after it was introduced in 1582, different countries were running different calendars, sometimes several conflicting ones at once, depending on which authority they answered to. A calculator can still produce a number for those dates, but calling it "the" weekday would overstate how settled the question really was at the time, so this tool sticks to a range where a single confident answer is honest.
Is the doomsday trick actually accurate, or just a rough shortcut?
Fully accurate. It is not an approximation or a memory aid that trades accuracy for speed, it is the same calendar arithmetic as any other method, just organised around dates that are easy to hold in your head. Done correctly it agrees with a calculator every time, which is also why the working shown above always matches the answer next to it.
Why does this differ from what a document written at the time says?
Almost always the calendar switch. A date from before your country adopted the Gregorian calendar, 1582 for much of Catholic Europe, 1752 for Britain and its colonies, considerably later elsewhere, was recorded using whichever calendar was locally in force at the time. This calculator answers on today's calendar throughout, which is standard practice but can disagree with a contemporary record by around one to two weeks either side of a switch.
Why do I get two days' difference between one year and the next sometimes?
A leap year in between. A date usually moves one weekday later each year, but if a 29 February falls between one occurrence and the next, the extra day pushes it on by two weekdays instead of one. The section above on why weekdays shift explains the arithmetic behind it.
What happens with a 29 February birthday in a non-leap year?
The birthday-planning table clamps it to 28 February for years that do not have a 29th, and says so next to the date, rather than skipping that year or guessing which nearby date was meant. Plenty of people born on 29 February celebrate on the 28th in other years for exactly this reason.
Does time zone affect the answer?
No. This calculator works from the calendar date you choose, not a specific moment or a server clock, so it makes no difference which time zone you are in.
This calculator answers on the modern Gregorian calendar throughout. For dates before your country's own switch from the Julian calendar, treat the weekday shown as the modern-calendar equivalent rather than a claim about what was written on a document at the time.