Doomsday Defense Game

Meteors fall toward the ground, and each one carries a date. One launcher stands below, over seven chambers, one for each day of the week from Sunday to Saturday, or Monday to Sunday if you prefer. Work out in your head which day the target's date falls on, and tap that day's chamber to fire its rocket. It is a game for practising the Doomsday rule, or any other way of finding the day of the week.

How to play

1. Choose the years the dates come from: this year only, 1900 to 2099, or any date from 1600 to 2099. The week starts on Sunday, and you can set it to start on Monday instead.
2. The lowest meteor is the target, with a ring round it. Tap another meteor to aim at it instead.
3. Work out the day of the week of the target's date, and tap that day's chamber. Its rocket is loaded and fired, and the right day destroys the meteor.
4. A wrong day fizzles. The whole launcher jams for 1.5 seconds, longer for each wrong day in a row up to 4.5 seconds, and says Jammed. The meteor drops a step and falls 1.5 times as fast from then on, or 2.25 times after a second wrong day. Guessing costs more than it saves.

The first wave has 4 meteors and each wave after it one more, up to 10. In the first two waves one meteor falls at a time, and after that two. In the first wave a meteor takes 19 seconds to reach the ground for a date this year, 28 seconds for 1900 to 2099 and 33 seconds for 1600 to 2099, and a little less in each wave after it, down to half that time.

When the game ends, the game over screen lists every date you got wrong or that landed, with its day of the week and the days you picked instead, so you can work them through again.

You have three shields, and a meteor that reaches the ground costs one. On a keyboard, press 1 to 7 for the chambers from left to right, and the left and right arrows to change the target. Escape or Space pauses.

Scoring

The sooner you know the day, the more a meteor is worth.

• A meteor destroyed with no wrong shot scores 100, plus up to 100 more for how high it still is.
• A meteor destroyed after a wrong shot scores 20.
• A wave with no wrong shot and no landing adds 250.

Your five best scores for each range of years are saved in this browser, with the wave each game reached. In practice, a wrong day shows the right one under the meteor, and a meteor that lands costs no shield, so the game goes on until you end it from the pause screen. Practice games are not saved.

How the Doomsday rule works

John Conway's rule rests on one fact: in any year, a handful of easy dates all fall on the same day of the week, the year's doomsday. They are 4 April, 6 June, 8 August, 10 October and 12 December, then 9 May and 5 September, 11 July and 7 November, 3 January, the last day of February, and 14 March. Find the year's doomsday, then count from the nearest of those dates to the one on the meteor.

• Each century has an anchor day: Tuesday for the 1600s and 2000s, Sunday for the 1700s, Friday for the 1800s and Wednesday for the 1900s.
• Take the last two digits of the year, add a quarter of them rounded down, and move on that many days from the anchor. For 2026 that is 26 and 6, 32 days, which is 4 weeks and 4 days, so the doomsday is four days after Tuesday: Saturday.
• In a leap year January's doomsday is the 4th, not the 3rd, and February's is the 29th.

With this year's dates you only need the last step, so start there. Our article on how to calculate the day of the week shows another method, with codes for the months and years.

Which calendar the game uses

Every date is in the Gregorian calendar, back to 1600. Britain and its colonies kept the older Julian calendar until 1752, so a British date from before then fell on a different day of the week at the time. The game counts it as if the Gregorian calendar had always been in use, as most calendar calculators do.

Try our other free memory games, such as Prime Factor Cannon for arithmetic or Capital Cannon for geography.