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The ] is for calculating the day of the week and makes use of the fact that in each year, The ] is a way of calculating the day of the week. It makes use of the fact that, in each year, the following dates are all on the same day of the week:


28/29 February (28/2 or 29/2) *] (or ] if it's a ])
4 April (4/4) *]
*]
6 June (6/6)
*]
⚫ 8 August (8/8)
⚫ *]
⚫ 10 October (10/10) and
⚫ *]
⚫ 12 December (12/12)
⚫ *]
⚫ *]
⚫ *]
⚫ *]


are all the same day of the week. This day of the week is called ] and is derived from the last day in February, be it the 28th or 29th. The algorithm also defines 9/5 and 7/11 and their reverses (5/9 and 11/7) to be Doomsday. This day of the week is called ] and is derived from the last day in February, which, depending on leap years, is the 28th or 29th. The dates listed above were chosen to be easy to remember; the ones for even months are simply 4/4, 6/6, 8/8, 10/10, and 12/12. The others (5/9, 9/5, 7/11, and 11/7) are based on the phrase "I work from 9 to 5 at the ]."


Therefore, if you know what day of the week Doomsday - last day in February - is for a given year, you can easily determine the day of the week for any other date in that year. Therefore, if you know what day of the week Doomsday — the last day in February — is for a given year, you can easily determine the day of the week for any other date in that year, by finding the nearest Doomsday.
⚫ The Doomsday ] was invented by ]. It can be used for either the ] or the ], but noting that Julian calendar Doomsdays usually occur on different days from the Gregorian calendar Doomsdays.


For dates in March, ] falls on Doomsday, but the pseudodate "March 0" is easier to remember, as it is necessarily the same as the last day of February.
⚫ ==An example==


⚫ The Doomsday ] was invented by ]. It can be used for either the ] or the ], but note that Julian calendar Doomsdays usually occur on different days from the Gregorian calendar Doomsdays.
In the year 2004, 29 February 2004 is Sunday
Hence Doomsday for 2004 is Sunday


⚫ ==An example==
Hence, we deduce that the following days in even months are sundays

4 April (4/4)
6 June (6/6)
8 August (8/8)
10 October (10/10) and
12 December (12/12)

For odd months remember 9/5 7/11. That means that the following are sundays too:

9 May (9/5)
⚫ 11 July (11/7)
⚫ 5 September (5/9)
⚫ 7 November (7/11)

For January and March, its quite different However. The Doomsdays are:

3 January (3/1) - for non leap years
4 January (4/1) - for leap years

and for March the doomsday is

7 March (7/3)


Suppose you want to know which day of the week ] ] is. In the year 2004, February 29 is a Sunday. Therefore, Doomsday for 2004 is Sunday. This means that April 4 is also Sunday, so April 18 is a Sunday as well (since it's two weeks after April 4). April 17, being the day before it, must be a Saturday.
As an example to find the day of the week for 17 April:
We know 4 April (4/4) is a Sunday,
So is (4+7) April,
And (4+14) April too,
That is 18 April is a Sunday, and hence
-17 April is a Saturday-


==External links== ==External links==
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Revision as of 06:29, 12 November 2004

The Doomsday algorithm is a way of calculating the day of the week. It makes use of the fact that, in each year, the following dates are all on the same day of the week:

This day of the week is called Doomsday and is derived from the last day in February, which, depending on leap years, is the 28th or 29th. The dates listed above were chosen to be easy to remember; the ones for even months are simply 4/4, 6/6, 8/8, 10/10, and 12/12. The others (5/9, 9/5, 7/11, and 11/7) are based on the phrase "I work from 9 to 5 at the 7-11."

Therefore, if you know what day of the week Doomsday — the last day in February — is for a given year, you can easily determine the day of the week for any other date in that year, by finding the nearest Doomsday.

For dates in March, March 7 falls on Doomsday, but the pseudodate "March 0" is easier to remember, as it is necessarily the same as the last day of February.

The Doomsday algorithm was invented by John Horton Conway. It can be used for either the Gregorian Calendar or the Julian Calendar, but note that Julian calendar Doomsdays usually occur on different days from the Gregorian calendar Doomsdays.

An example

Suppose you want to know which day of the week April 17 2004 is. In the year 2004, February 29 is a Sunday. Therefore, Doomsday for 2004 is Sunday. This means that April 4 is also Sunday, so April 18 is a Sunday as well (since it's two weeks after April 4). April 17, being the day before it, must be a Saturday.

External links

Category: