Calculate the day of the week for any date in your head

Re:Calculate the day of the week for any date in your head by sverhagen on Wednesday January 01, 2020 @01:34AM (#21926942)

For all but the most basic details my rule is that if I have to remember it, the system is broken. I know a small handful of passwords off the top of my head, a few email addresses, my wife's and my phone numbers, and the phone number of my home during childhood. (I'm obviously exaggerating, you know, for dramatic effect.) This trick requires so many mnemonic devices that it makes my head spin. It's still impressive if you can do it. Just not for me.

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Re:Calculate the day of the week for any date in your head by netsharc on Wednesday January 01, 2020 @03:46AM (#21927263)

You only need to remember 5 things:

- For 2020, the doomsday is Saturday.

All the dates below are doomsdays (So this year they are all Saturdays):

- 4/4, 6/6, 8/8, 10/10, 12/12, easy enough

- 9/5, 5/9, 7/11, 11/7 (9-to-5 at 7-11)

- 0th of March (last day of February)

- 3rd of January for non-leap years, and 4th of January for leap years. So this year the 4th is Saturday.

And to calculate dates: mod 7.

So e.g. 7th of July: it's is 4 days before Saturday 7/11, so it's a Tuesday.

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Re:Calculate the day of the week for any date in your head by stkdump on Wednesday January 01, 2020 @06:07AM (#21927593)

You can skip the leap year info completely if you consider both January and February as belonging to the previous year. That makes sense as the additional day (Feb 29) is at the end of Februry.

Last year Pi Day was Thursday, this year Saturday.

So the general rule then is: March 14th, April 4th, May 9th, June 6th, July 11th, August 8th, September 5th, October 10th, November 7th, December 12th, January 2nd, February 6th are Pi Days.

Edit: another nice rule is that usually Pi-Days increase by 1 each year (2 in leap years), so next year it will be Sunday, then Monday, Tuesday, Thursday (leap year), Friday, etc.

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Re:Calculate the day of the week for any date in your head by netsharc on Wednesday January 01, 2020 @12:32PM (#21929143)

But why do you call them Pi-Days? Pi Day is 3/14 because that's somewhat similar to 3.14, which is Pi to 3 significant digits..

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Re:Calculate the day of the week for any date in your head by stkdump on Wednesday January 01, 2020 @01:13PM (#21929468)

They are the same weekday as 3/14 and it makes a little more sense than 'Doomsday', which the article chooses.

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Re:Calculate the day of the week for any date in your head by netsharc on Wednesday January 01, 2020 @02:47PM (#21930087)

I vehemently disagree, you can't just redefine Pi-Day to what you want. Pi Day is 3/14, not 4/4 or 6/6.

As for calling it Doomsday, well, it's from the title of the paper about the algorithm, by the man who invented it, the famous John Conway: https://en.wikipedia.org/wiki/Doomsday_rule

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Re:Calculate the day of the week for any date in your head by stkdump on Wednesday January 01, 2020 @11:52PM (#21933119)

I didn't come up with this. According to the article, Roman Weil of Princeton teaches the same.

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Re:Calculate the day of the week for any date in your head by karaterobot on Wednesday January 01, 2020 @12:12PM (#21929008)

This comment made me take a brief inventory of some of the systems I have internalized to the point where I don't think about them as being memorized: the rules of the road while driving, the conventions of UNIX-style commands, unit conversions, hotkeys in (and across) applications, how street addresses and road navigation works, how to use a search engine, how to compose a sentence in English, and so on.

At some point in my life, all of these seemed needlessly complex and arbitrary to me, but they don't anymore (at least unless I think too hard about them). I don't think about them unless I have to teach them to someone else, at which point I start thinking about how seriously broken they are.

I suppose that if I needed to get the day of the week from any date in history, and used this algorithm often enough, I'd eventually think of it as something I didn't have to memorize either.

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Re:Calculate the day of the week for any date in your head by BrandoElFollito on Wednesday January 01, 2020 @04:08AM (#21927324)

So do I.

Except my wife number, which I should learn. But I know how to say if a month is 30 or 31 days by counting my knuckles.

These are the kind of "hardly important but easy to remember tricks" I like

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