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## Calendar Fun

Published on Sunday, December 30, 2007 in , , , , , , , ,

Since we're about to switch over to a new calendar, how about some new calendar fun to go with it?

As many of you know, you can learn to give the day of the week for any date, and even practice the feat, right here on Grey Matters. If you want to make the work easier, there's my specially-prepared Day For Any Date Routine 2008 calendar, too.

The fact that calendars always have numbers in order and increase by 7 from week to week makes makes many mathematical tricks possible. A simple example would be having someone circle any 3 days in a row. Once they've done that, tell them to add up the 3 numbers they circled, and to give you the total. When you get the total, simply divide it by 3, and that will tell you the middle date. The other two dates, of course, are the ones immediately before and after it. If they gave you a total of 48, you divide it by 3 to get 16. This tells you that they chose the dates 15, 16 and 17. This will also work with columns, too, but you'll have to add an subtract 7 for the remaining two days. If they give you 57, you divide it by 3 and get 19, which tells you that they chose 12, 19 and 26!

What about other arrangements of numbers? Here's an article that starts by teaching you a simple calendar feat with a square group of numbers, and proceeds to show you how to design your own custom calendar tricks using the same approach! If you understand the algebra well enough, you might be able to work out this trick before clicking on its secret.

Instead of trying to find out chosen dates from a total, you can go the opposite direction, too. Here are two impressive routines, in which you apparently add up a group of numbers from the calendar faster than your audience member can with a calculator.

What if you want to make a calendar routine seem more magical than mathematical? If you have Java installed in your browser, try out this bit of Calendar Magic from Cut-The-Knot.

Martin Gardner's Mathematics, Magic and Mystery has a great section on Calendar Magic. If you click on that last link and scroll down, you'll see one of the most magical calendar routines by Walter Gibson (creator of The Shadow!), called Gibson's Circled Dates. This routine involves circling dates that are unknown to you, yet you're still able to give their total. Milbourne Christopher wrote up a great stage presentation for this feat called Super Date Sense.

Back in the May 1999 issue of MAGIC Magazine, Bob Farmer published his version of a calendar idea originally developed by Robin Dawes. Instead of a lightning calculation or a magic trick, this takes the form of a game. A marker is placed on January 1st of any calendar, and two players take turns alternating moves. There are only two legal moves. A player can either move the marker to any later date in the same month, or the same date in a later month. The person who moves the marker onto December 31st is the winner.

Try playing this game against the computer. After a few games, you'll realize that the computer is winning every time. If you study the moves made by the computer closely enough, you'll be able to work out why those moves are chosen, and why they win the game every time. You can even play the game backwards, and only a minor alteration in the math is required to win every time. Here's one hint: The math required to win every time at this calendar game is so simple that, once you've mastered it, you can play the game with no calendar at all, and just call out the dates verbally!

Now you know myriad ways to amuse, amaze and even cheat with a calendar. Go out, have some fun with these, and have a happy new year!