You must have seen several talent-shows where people would guess the day of a given dates. Some of you must have thought they do it by memorizing the entire calendar. Well, it somewhat does require memorizing, not the entire calendar though, and the rest is basic calculation. Let’s jump right into it.
The method that
we are gonna discuss about is called Doomsday Rule. This algorithm was devised
by John Conway, inspired by Lewis Carroll’s perpetual calendar algorithm, in
the year 1973. By using this method anyone can calculate the day of any date in
any given year.
In this method one need to remember these special
dates also known as doomsday, these days include:
4/4,6/6,8/8,10/10, 12/12, 7/11,11/7, 9/5,5/9
[ Note- Every dates in this article are in the format
of month/day/year]
The date for the month of January and February differs
according to the leap year.
The special dates of January are the 3rd
and 4th (for leap year) and for February, it is 28th and 29th
(leap year). So, it is 1/3 and 2/28 for three years and ¼ and 2/29 the other.
For March, the special date is the pi day, that is,
3/14.
Other notable special dates are 10/31- Halloween and
12/26- boxing day.
Now, one may ask what’s special about these dates, to
that, these days in a given year appears on the same day. For example, if you
know the 12/12 is Friday, then the rest of the dates mentioned will also be on Fridays.
Now that we know the trick let’s learn about how to apply
them. For example, let’s find out the day of 11/26/2023.
For every question we need a reference date, which
could be any of the doomsday. For this case let’s take the pi day as the reference.
Once we know about pi day we will automatically know about the rest of the days.
And that’s when we face the question that how do we know that. There is a
simple trick to that as well. All you need to remember is that the pi day was a
Tuesday in the year 2000.
2000 - Tuesday
2001- Wednesday
2002- Thursday
2003- Friday
2004- Sunday
As you can see, that every year the day increments by
one and in the leap year it is incremented by 2.
When you need to find out about past dates, those of
before 2000, all you need to remember is this pattern,
1700- Sunday
1800- Friday
1900- Wednesday
2000- Tuesday
And this pattern repeats, which means 2100 pi day
would be on Sunday, 2200 on Friday and so on.
Now let’s get back to the question. So, the year is
2023. We know on 2000 pi day was in Tuesday, all we gotta do is add the number
of years plus the number of leap years to Tuesday,
Tuesday + 23 + 23/4(=no. of leap years)
Tuesday + 23 + 5 (ignoring the decimal, which is to be
done in every case, since years can’t be in decimal)
Tuesday + 28= Tuesday (*since 28 is a multiple of 7)
(*when days are added with numbers which 7 or multiple
of 7, you would get the same day)
So, pi day in 2023 is a Tuesday. After this step you
need to find the nearest doomsday to the date in question. Here, it would be
11/7. 11/7 is also Tuesday. To find 11/26 we gotta add
Tuesday + (26-7) = Tuesday + 19 = Sunday
Hence 11/29/2023 will be a Sunday.
You can go to the calendar and check if the results
are correct. You can try this method on any date and year and see it for
yourself.
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