Wiktionary
half-birthday
n. (alternative form of half birthday English)
Wikipedia
Half-birthday
A half-birthday is a day approximately six months before or after the anniversary of a person's birth. It is sometimes marked by people whose birthday falls near major holidays, the celebration of which may overshadow celebration of the birthday. It may also be marked by students whose birthday does not occur during the regular school year; a half-birthday allows a celebration with friends at school.