Mentally Calculate The Day Of Any Date With The Doomsday Rule

# Mentally Calculate The Day Of Any Date With The Doomsday Rule

When someone professes to have a photographic memory, the first thing many people ask them is to name the day of the week on which a random date landed. Often, they do it easily, and everyone around is sufficiently impressed with their memory. Unfortunatelyâ€”or fortunately, for those without such a giftâ€”naming the day of the week of a date doesn't require a photographic memory; just some quick mental arithmetic. Anyone can do it using the doomsday rule, a simple algorithm devised by mathematician John Horton Conway.

Related: Should The Year Have 13 Months?

Related: Use The Rule Of 72 To Estimate When Your Money Will Double

Here's how it works: in any given year, a handful of dates land on the same day of the week. That day is what Conway calls the "doomsday." For 2015, the doomsday was Saturday; for 2016, it's Monday. (See the video below for more on how to calculate a year's doomsday). Some of those dates are easy to remember: 4/4, 6/6, 8/8, 10/10, and 12/12 all land on the doomsday. It takes an easy mnemonic device to recall some others: 5/9, 9/5, 7/11, and 11/7 can be remembered with the sentence "I work 9 to 5 at the 7-11." There are also a few one-off doomsday dates: March 14th (3/14 or Pi Day, for those who use the day-month format), the last day in February (regardless of leap year), and January 3rd or 4th, depending on if it's a leap year.

Once you have those dates memorized, you can calculate the weekday of any other date via its proximity to a doomsday. For example, what weekday did Dame Judi Dench's birthday, December 9, land on in 2015? That year's doomsday was a Saturday, so we know that 12/12 landed on a Saturday. The 9th is three days before the 12th, so Judi Dench's 81st birthday landed on a Wednesday. Still confused? Watch the video below to clear it up.

### Calculate A Date's Weekday With The Doomsday Rule

If the article above had you puzzled, this video should clear up any questions.

05:08

from MindYourDecisions

Key Facts In This Video

• 1

Simply typing a date into the website WolframAlpha can tell you which day of the week it falls/fell on. (0:08)

• 2

The doomsday rule states that certain days of the year will always fall on the same day. (0:50)

### The Origin Of The Modern Calendar

Without it, we wouldn't have the doomsday rule.

06:17

from Today I Found Out

Key Facts In This Video

• 1

The ancient Egyptians used Sirius to predict the annual flooding of the Nile, allowing them to become one of the first civilizations to use a solar calendar rather than a lunar one. (0:34)

• 2

Julius Caesar started the more precise Julian Calendar in 45 B.C. (2:07)

• 3

The Mayan calendar consists of 18 months of 20 days each, with five extra days at the end of the year. (5:20)

### The Easiest Way To Calculate Pi

Mental arithmetic it ain't, but this is still a brilliant way to calculate pi to a large number of digits.

01:51

from Tipping Point Math

See all
Get smarter every day! Like us on Facebook.
You'll get the most interesting and engaging topics in your feed, straight from our team of experts.