Before I say hello this time I need to define a few things:

d = day of the week (Sunday = 1, Monday = 2 … Saturday = 7)
w = week of the year (This is week #28)
y = year of the century (2012 = 12)
c = unnecessary for most of you
! = not a factorial symbol, just punctuation

Those of you who will be surviving this century are going to have to find yourselves a new equation for calculating what day of the year it is, sorry.  I’m just joking, those of you who are going to be alive in 2100 can just change ‘y’ to equal ‘year of the millenium’ and you’ll be good to go.

Ok.  I think I’m ready now.

Happy day of the year number d + 7 * ( w – 1 ) + 6 – [ y + ceiling ( y / 4 ) + 5 ] mod 7!

How exciting.  That equation will work until the last day of 2099.  Think about that for a second.  If you are having a baby this year sometime your child will turn 87 before this equation expires.  That should be mildly comforting for any autistic readers and just a little bit disturbing to the rest of you.  Sucks to not be autistic sometimes.

NOTES:
Ceiling means to round up no matter what, ex. ceiling ( 5.2 ) = 6
Mod is the remainder of a number when divided by another number, ex. 25 mod 7 = 4