Before you begin

There are two preparatory steps to take before calculating an actual date.

1.  Memorize a twelve digit number.
Believe it or not, perhaps the most challenging step of the whole process is to memorize the following twelve-digit number.  I've broken it into four groups of three digits; you'll spot a pattern in the last three groups:

622 - 503 - 514 - 624

Each digit corresponds to a month of the year.  January gets a value of "6", February gets "2", and so on.

2.  Learn to "drop the sevens" (modular division).
The algorithm is based on counting the total number of days relative to an absolute date (I use January 1, 2000 as the absolute reference point).  So you don't really need to keep track of the total number of days (or weeks), you only need to track the remainder after dividing the total number of days by seven.  If you didn't understand all that, it's ok - the formula works just the same.

To "drop the sevens" from a number, you simply subtract from that number the highest multiple of seven which is less than the number itself.

Examples:

• 9 is equal to 7 + 2.  After "dropping the sevens" from 9, you have 2. 

• 37 is equal to (7x5) + 2.  Anyone familiar with basic multiplication tables should recognize that 35 is a multiple of seven.  So, after "dropping the sevens" from 37, you have 2.

• 20 is equal to 14 + 6.  14 is clearly the largest multiple of 7 which is less than 20, so after "dropping the sevens" from 20 you are left with 6.

With a little practice this becomes quite easy.  Also, for purposes of this calendar calculation method, you will never need to "drop sevens" for a number larger than 100, so you only need to recognize multiples of seven up to 7 x 14.