# What Day Is Doomsday? How to Mentally Calculate the Day of the Week for Any Date

The so-called doomsday algorithm uses clever mental arithmetic and mnemonic tricks to enable a quick determination. Trick question: What day did September 6, 1752, fall on?

The end is not nigh
Despite its usefulness in retracing failed predictions by Camping and other prognosticators of end times, the doomsday algorithm has nothing to do with the apocalypse. Conway named his calendar algorithm after discovering an interesting property of a certain day of the week in a given year, which he called "doomsday." But his use of "doomsday" is probably a misnomer. One could just as easily refer to it as "anchor day."

The idea that makes the doomsday algorithm tick is that certain memorizable dates always share the same day of the week within any given year. For example, April 4, June 6, August 8, October 10 and December 12 all fall on a Monday in 2011. Moreover, you can check on a calendar that these dates all fall on the same day of the week in any year. For instance, in 1994 they were all Mondays, too. Conway used this fact as the basis for his algorithm. This special day of the week, shared by 4/4, 6/6, 8/8, 10/10 and 12/12, is the anchor day. (Which day of the week anchor day falls on varies from year to year.)

Other memorizable dates also fall on a given year's anchor day: 5/9, 9/5, 7/11 and 11/7 are all Mondays in 1994 and 2011. Just as 4/4, 6/6, 8/8, 10/10 and 12/12 are easy to remember because of repeating digits, one can easily remember the four additional dates using Conway's simple mnemonic: "Working 9 to 5 at the 7–Eleven." Armed with these memorizable anchor dates, one can easily determine the day of the week for nearby dates. (There are other tricks to assist with dates in the first months of the year—for example, the last day of February is always an anchor day, leap year or no.) For instance, since we already know that 9/5 was a Monday in 1994, it is easy to deduce that September 6, 1994—Harold Camping's no-show apocalypse—was a Tuesday. The use of a nearby anchor is the key to the doomsday algorithm.

The drifting anchor
The day of the week drifts annually for a given date. October 21, 2010, was a Thursday. October 21, 2011, will be a Friday and—assuming that the world does not really end that day—in 2012 October 21 will be a Sunday and October 21, 2013, will be a Monday. There must be a pattern here. Indeed, the day of the week for a given date progresses by one annually, except on leap years, where it progresses by two. Moreover, there is a simple mathematical formula for measuring the cumulative annual drift of the day of the week from the start of the century. Although the formula is simple, it is difficult to solve without pencil and paper. Lewis Carroll discovered an alternative formula for this calculation in 1887, which looks more complex but is actually more suitable for mental calculation. In fact, Conway reused Carroll's method as part of the doomsday algorithm.

Last year Michael K. Walters of the Air Force Institute of Technology and I came up with a different way of calculating the cumulative drift of the day of the week. We believe that our method is easier to perform mentally than Carroll's, because our method requires fewer divisions and memorizations. We call our method the Odd+11 method. A summary of our method is available as a flowchart here.

So what day of the week did September 6, 1752, fall on? To answer that question, one needs to know a bit of history on our current calendar system. Earth spins around its axis approximately 365.24219 times annually. This rate is known as the mean tropical year. The true number of days in the mean tropical year actually varies slightly over time. Our modern calendar system of 12 months started with the Julian calendar circa 45 B.C. The Julian calendar approximated the mean tropical year as 365.25 days and proposed 366-day leap years for every one divisible by four. This approximation made the Julian calendar fairly accurate in tracking the annual seasons for several centuries. But eventually the approximation errors accumulated. By 1582 the Julian calendar was out of sync with the seasons by several days. Pope Gregory XIII mandated a reform to the calendar; his Gregorian calendar approximated the mean tropical year as 365.2425 days. The pope thereby amended the rule for determining leap years: A year divisible by 100 is only a leap year if it is also divisible by 400. For example, the year 2000 was a leap year, but the year 1900 was not, because when divided by 400, it does not render a whole number.

The Gregorian calendar was not widely adopted across the world until much later. In fact, the British Empire and its colonies did not use the updated calendar system until 1752. By then the Julian calendar was out of sync with the seasons by 11 days. In order to synchronize with the Gregorian calendar, it was mandated that Wednesday, September 2, 1752, be followed by Thursday, September 14, 1752. Hence, the date September 6, 1752, does not exist in British and American history! A nonexistent date certainly does not have a day of the week. If you think that daylight saving time is confusing, try skipping days on for size.

Chamberlain Fong is a computer engineer living in San Francisco. He has degrees in mathematics and electrical engineering. He grew up obsessively reading Martin Gardner's columns in Scientific American.

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1. 1. ZoeyKay 06:00 PM 10/18/11

I did multiple calculations using 2 different formulas, and they all work for last century (1900=Wednesday), but for this lovely new century, there needs to be a slight shift to 2000=Tuesday, and then all of the calculations seem to work. Don't know how it will play out for the next century - I'm pretty sure I won't be here for that, unless there are some MAJOR advances in medicine. Please correct me if I'm wrong.

2. 2. photainam 08:03 AM 10/19/11

How about using mobile device to find out the same results:) Victorian methods would be 'cool' during that period only lol. In the same vein of thought: It's laughable to see Christmas store decorations with mechanisms dated back to the Victorian periods also... it's no longer a novelty, it's rather nostalgic perhaps. Modern day people need to be stimulated with the wow factors with all the new technical wizardry of the day:)

3. 3. jh443 08:44 AM 10/19/11

I am extremely disappointed in this article. I'm considered by my friends and acquaintances to be rather adept at math, but there's no way I could do this algorithm in my head. I agree with photainam who basically said, "There's an app for that."

4. 4. jestephens 10:21 AM 10/19/11

Maybe this is a quibble. The author writes that Lewis Carroll "dabbled in mathematics" while writing his classic works. I had understood that Carroll taught mathematics at Oxford until the 1880s. "Dabble" would seem to imply some casual or superficial attention to mathematics. It would seem, instead, that teaching mathematics was his occupation while he wrote very successfully. No real dabbling in either field of endeavor.

5. 5. janand712 02:08 PM 10/19/11

My uncle, Donald Barta, has been doing these calculations in his head for years, and always has a correct result in seconds. He is a retired rocket scientist, though.

6. 6. powerslide 05:09 PM 10/19/11

Wouldn't you know it ... Lewis Carroll's algorithm also made an appearance in Scientific American Supplement #598 dated June 18, 1897, and published in New York.

Thank you Project Gutenberg!
http://www.gutenberg.org/files/11662/11662-h/11662-h.htm#10

"TO FIND THE DAY OF THE WEEK FOR ANY GIVEN DATE"

7. 7. powerslide in reply to ZoeyKay 05:40 PM 10/19/11

Each century has a certain centennial anchor day. It is useful to memorize them for evaluating the Doomsday algorithm for practical dates.
1800 -> friday
1900 -> wednesday
2000 -> tuesday
2100 -> sunday

The pattern repeats itself every 400 years with only these 4 possible values. That is, 2200 has centential anchor day on friday, too.

Or if you like to do more calculations, you can use a formula provided by Yingking Yu:
http://improvedddabyykyu.blogspot.com/

8. 8. himadri1 09:05 PM 10/19/11

"Earth spins around its axis approximately 365.24219 times annually", the article says. Isn't it 366.24etc ? There is a difference between how many spin around its own axis and how many days in a year. This is because, earth goes around the sun once a year. Just imagine the earth does not spin at all and goes around the sun. That would give us a -1 day per year. Minus because sun would rise in the west. So, zero spin gives -1 day. 366.25 spin gives us 365.25 days.

9. 9. powerslide 06:56 PM 10/20/11

Here are some interesting day-of-the-week quizzes from Professor Sidney Graham of Central Michigan University
(excerpts from http://www.cst.cmich.edu/users/graha1sw/pub/doomsday/Doomsday.html)

a) In the movie "Demolition Man", the date August 3, 2032, is identified as a Monday. Is this correct?

b) The movie "The Gunslinger" has been featured on "Mystery Science Theater 3000." In the movie, the words "Friday, May 21, 1878" are imposed on the opening scene. Did the scriptwriter get the day of the week correct?

c) In the book Fried Green Tomatoes at the Whistle Stop Cafe, an elderly woman named Virginia Threadgoode recalls events from her distant past. On page 12, she states, "Some people thought it started the day she met Ruth, but I think it started that Sunday dinner, April the first, 1919, the same year Leona married John Justice." Explain why this recollection is in error.

d) The "Star Trek: Deep Space Nine" episode entitled "Past Tense" is set in the 21st century. In one scene, a calendar shows the date as Friday, August 30, 2030. Is the day of week correct?

e) Shakespeare and Cervantes both died on April 23, 1616. Yet Shakespeare died on Tuesday and Cervantes died on Saturday. Explain.

http://www.cst.cmich.edu/users/graha1sw/Pub/Doomsday/DoomsdayBloopers.html

10. 10. Baraka Tales_Kween KleoKatra 11:55 AM 10/22/11

According to this Doomsday has been canceled
http://vimeo.com/18814554

11. 11. spectral 01:56 AM 11/2/11

Here is a fascinating video of mathemagician Arthur Benjamin performing day-of-the-week calculations. He's probably using some variant of the doomsday algorithm.

12. 12. spectral 12:25 AM 11/28/11

another video featuring Prof. Arthur Benjamin and his day-of-the-week calculations:

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