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1.
Birthmorrow - often mistaken for birthday in advance.
this obviously incorrect however...
If "today" is the "birthday", then "birthmorrow" is the day AFTER the "birthday"...
Sorry I missed your birthday. Oh, this present is for you. Happy Birthmorrow!
by dat-guy March 20, 2012
 
2.
Wishing someone Happy Birthday, a day in advance.

Birthday = Birth + today - to
Birthmorrow = Birth + tomorrow -to
Happy Birthmorrow, Sunshine!

Hey Rachel, I am out tomorrow, so Happy Birthmorrow to you!
by Shar6 October 08, 2010