How to Figure Out the Day of the Week For Any Date Ever

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Want to go NEXT LEVEL? Learn how to adjust for Julian calendar and BC dates

You might think that computers are the only things that run algorithms, but you're wrong. Here's a neat mental trick for calculating the day of the week for any day ever, developed by famous mathematician John H. Conway

READ MORE:

Martin Gardner, "The Universe in a Handkerchief: Lewis Carroll's Mathematical Recreations, Games, Puzzles, and Word Plays"

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It's Okay To Be Smart is hosted by Joe Hanson, Ph.D.
Director: Joe Nicolosi
Writer: Joe Hanson, Ph.D.
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Producer: Stephanie Noone and Amanda Fox
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[besmart]

Sure, you could just Google it… but where's the fun in that?!

I promise, if you can do arithmetic, you can learn this trick. Let me know how impressed your friends are!

[jmack]

Before the video really gets going: "awesome i'm going to learn this!!"

5 mins in: "well nevermind..."

[eduardogomez]

2 and a half years later, I'm here, almost 3am in the moorning, I finally managed to do this on my own.
Now I just have to wait that covid disappears to use my new skill at parties. 😎

[nindoninshu]

"Congratulations, you just used an algorithm!" Uh no... You did...

[MyHansSarpei]

2min into the video and already confused af

[mr.fluffyface]

You assume I can do division in my head...

[OctorokSushi]

Okay watching this at 3:23am was a mistake lol I'll try to learn this tomorrow.

[tjgrembowski]

For those who are/were as confused as I was: The 4 numbers that you are determining (we'll call a, b, c, & d) are respectively "a=century code", "b=how many times 12 goes into the last 2 digits of the year", "c=the remainder of the multiple of 12 from the last 2 digits of the year", and "c=how many times 4 goes into the remainder".

On a leap year - the ONLY thing that changes is the doomsday for January and February. All other math stays the same. Also it should be noted that when doing a "remainder" of a year that 12 goes into "0" times, the remained would be "x-(12x0)" where x equals your last 2 digits of the year and 12x0 is obviously 0. That being said:

1987 - Not a leap year. a+b+c+d mod 7 = Doomsday.
- a = 3, b = 7, c = (87-84 = 3), d = 0.
- 3 + 7 + 3 + 0 = 13
- 13 - 7 = 6
- Doomsday is Saturday. January 3rd is Saturday. April 4th is Saturday

2004 - Is a leap year. (Jan. and Feb. are different doomsdays, but math stays the same).
- a = 2, b = 0, c = (04-(12x0)) = 4, d = 1.
- 2 + 0 + 4 + 1 = 7
- 7 - 7 = 0
- Doomsday is a Sunday. Since it's a leap year - We use January 4th (instead of the 3rd) and February 29th (instead of 28th). All other dates will stay the same. Therefore: January 4th is a Sunday, April 4th is a Sunday, and June 8th is a Tuesday.

I really hope this helps someone out. I may make a follow up video for this!

[JuQmadrid]

I think I'll go to play something more on my intellectual level. Anyone want to join me in a Hungry Hungry Hippos game?

[carlosdealcantara_]

Guys, I barely have words to describe what I am feeling right now. I`m so excited!!! I have just talked to a bunch of friends finding out the day of the week of their date of birth. Thank you so much and congratulations for this awesome video. <3

[fartrack]

i did it after practicing it for 3 hours and watching this video countless times i am able to do it man i thought i would never do this but i can its just about patience

[diegotejada]

No! This is _my_ thing! What will be my party trick now? ...just kidding, I'd never be invited to a party.

[sebastianelytron]

Party tricks to impress the ladies.... this channel never ceases to amaze me.

[bigdickpornsuperstar]

NOPE! Nope... nope... nope... nope... nope...

I didn't live long enough to reach the computer age to go back to doing maths in my head.

[Wonderouz]

1:33 my brain🤯! My damn head literally starting hurting trying to figure this out

[nmlss]

Wow, I understood absolutely nothing at all!! Still, great video. Somehow, interesting.

[korrectmethod]

It finally clicked after about an hour of focusing on how to do it. Your leap year number with the thumb confused me, but i realiszd later you were thinking ahead and saving the step of going through the days in your head. Impressive shortcut but it confused me as a student. The cheatsheet was insanely helpful to memorize the steps. I've practiced this for probably 7 hours over the past 2 days to get quicker at it, and I can't wait to do nothing with this skill. :) Thank you.

[latefordinner]

I thought this was going to be an easy trick like using your knuckles to find out which months have less than 31 days

[MickyVideo]

Oh, so that's how the dude at TED did it.

[sjoerdwennekes]

“There is a non-US version because I know we write dates funny”.
As a non-American; thank you!! I will not rest until I have learned this algorithm!