How to figure out the day of the week by the date
 

Ignore the goofy url, this is a really fascinating mental exercise. I just started reading it, but i'm definitely going to try to commit it to memory:

http://www.h2limousine.com/calendar2.php

Basically, the day of the week (1=sunday, 2= Monday...0=saturday) is:

(Month code + year code + day of the month) mod 7.

Month codes:
January: 1
February: 4
March: 4
April: 0
May: 2
June: 5
July: 0
August: 3
September: 6
October: 1
November: 4
December: 6

Year codes:
2005: 5
2006: 6
2007: 0
2008: 2
2009: 3
2010: 4

The 'mod' operation means 'subtract seven until you can't subtract seven any more without getting a negative number. Thus, Mod 25 would be [ 25-7 (18) - 7 (11) - 7 ] =4

A practical example. My sister's birthday is April 15th. Her 21st is this year, and I'm curious whether it's on a weekend, as she wants me to come out.
Month: 0
Year: 2
Day: 15

17 mod 7 = 3. d'oh, it's a Tuesday.

That's the cliff notes version, but check out the site if it at all interests you :thumbsup:

Anyone else have any cool math tricks? I class this about three times as easy as memorizing a substancial number of digits of pi (which I still have out to 3.141592653589793238...used to have more), and thirty times as useful :)

Pretty cool....

Wouldn't it be easier to just look at a calender?
Mine says that you are correct. Your sister's birthday is on a Tuesday.

Once you memorize the month codes, and, at most, this year and the next, you can do this on the fly, in your head, anywhere, extremely fast.

So, probably yes, if you have a calendar in front of you, and it's already open to the right month. If you have to flip for the month, probably not. If you don't have a calendar, definitely not.

you double click on the clock setting in the right hand corner of your desktop and u can find out like that! no need for finger calculations :D

Maybe a useful mental exercise...I tend to use my Palm for this type of thing...which may or may not be sad, depending on who you ask.

Great party trick.

That appeals to my inner geek :thumbsup: Unfortunately, I suffer from CRS and would probably never be able to commit those month/year values to memory :(

As an aside, the mod function is the remainder of the formula if you apply long division to it. In your examples:
7 into 25: 3 remainder 4 (3 x 7=21, 25-21=4)
7 into 17: 2 remainder 3 (2 x 7=14, 17-14=3)

If you grew up learning long division, it might be a wee bit easier to do it in your head that way as opposed to trying to do the subtraction steps.

/geek

I've decided to commit this process to memory. This involves three steps.

1. Associate pictures with each number, 0 through 9 (I went ahead and did 0 through 10)
2. Associate pictures with each month
3. Assemble these pictures into twelve scenes, one for each month, involving each month and code's number.

So far, I've memorized images for each number. It was super easy. Byrnison, try it! It only took me a lunch today do, and I'm slightly drunk and still remember them all. I just made these up, feel free to use your own list. I saw some lists online that looked pretty lame...remember, you're going to have to put these objects in scenes to remember other stuff...having a 'baseball team' for number 9 just seems unwieldy and not very flexible. (eg, I'd rather imagine a cat in a bucket for 95 than a baseball team in a bucket). There are a lot of tricks you can do with this list once you have it down, not just this one. I'll probably start a new 'memory tricks' thread once I get more time to research them.

Here's mine:

1. The one ring from lord of the rings
2. Two penguins (who mate for life)
3. A tree--sounds like three, and 'three on the tree' from old motorsports resonates
4. Door. Sounds like four, my picture has four panels, and i'm on the market for a '4-door' aka sedan
5. Bucket...a five gallon bucket
6. Gun. specifically a Six Shooter
7. Dice. Seven is the most common dice roll, also 'seven come eleven!'
8. Ball. Magic 8 ball says, this will be easy to remember
9. Cat. Cat o' nine tails or cats with nine lives, depending on whether i'm in a kinky mood or not
10. Tin can. Ten sounds like tin, plus my image is a tin can radio, with one can standing up and one laying down that looks like a ten
Zero. An egg. I knew someone who used "The ol' goose egg" as slang for zero or nothing, plus they're both round.


Once you pick out your ten (or eleven) images, find clipart that makes you happy for each one, and print that sheet. Study it a few times, then flip it over and try to draw all 11 pictures from memory. Then do it again. And again. Three times was enough for me to have them pretty permanently etched.

Tomorrows task: Month pictures (and more drilling on the number pictures to make sure they stuck).

Going to try this. I've always been fascinated with those savants who can do calendar calculations - I wonder if their internal method is based on this at all?

I'd say devote a day to each stage of the image memorization. I still have my number pictures :thumbsup:

Now workng on my set of month pictures.

OK, got my month pictures:

January: Cell phone (CES is in January every year)
February: Heart
March: Drum major's hat
April: Stack of coins
May: Flowers
June: Squirt gun
July: Uncle Sam's Hat
August: Cake (my birthday!)
September: Backpack (full of school books)
October: Jack o'lantern
November: Turkey
December: Present

My month codes that I need to memorize, therefore, are:

Cell phone + Ring
Heart + Door
Drum Major Hat + Door
Stack of coins + Egg
Flower + Penguins
Squirt Gun + Bucket
Uncle Sam + Egg
Cake + Tree
Backpack + Revolver
Pumpkin + Ring
Turkey + Door
Present + Revolver

The year codes are in order the next three years, so at least for then, I can 'cheat' the memorization of those.