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1.
You're birthday but half the way inbetween your birthday from the previous and next year (:
WOOOOH! My half birthday in a week!
by yapdizap April 04, 2009
 
2.
a day approximately six months before or after the real anniversary of a person's birth. Typically celebrated by people who feel they are twice as important as everyone else or people who want to get more presents by coming up with a lame excuse.
Hey, Roshni's half-birthday is tomorrow, she's turning 21 1/2, should be get her something?

No - because it's a made up thing.
by Da Zhong October 11, 2011