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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, with half a cake.
There are two ways to calculate half-birthdays.
The easier but potentially less precise method is to take the number of the date of the birthday and advance the month by six, e.g. December 5 becomes June 5. Because not all months have the same number of days, this does not always work – for example, six months after an August 30 birthday would be February 30, which is nonexistent in the Gregorian calendar.
In the U.S., there is a 10% federal income tax penalty for making an early withdrawal from a IRA, 401K, annuity, or whole life insurance policy before age 59½, with some exceptions, and this is the method used. If it results in an nonexistent date, the last day of the month is used instead--even if it is a day or two short.
The more precise method is to add or subtract half the number of days in a year to the birth date. In the case of a common year, this would be 182.5 days. In leap years, the number of days would be 183. This method would lead to a March 1 or February 29 half-birthday for an August 30 birthday, depending on whether it's a leap year.
The practise of celebrating half-birthdays is not without controversy, some parents have commented that it amounts to pandering spoiled children and attempting to social one-up other parents. At least one party planner has stated that 'They're ludicrous,' she admitted. 'And I'm seeing them happening more and more - the excuse is always, "Oh, but my child has a winter birthday and so the kids miss out on a bouncy castle in the garden. It's madness!' 
Some parents and educators have called half-birthday parties “out of control” and have used them as an example of parents putting unnecessary pressure on themselves and altering their children’s expectations of reality.
At least three children's books have been written about half-birthdays:
- Pomerantz, Charlotte (1984). The Half-Birthday Party. New York: Clarion Books. ISBN 0-89919-273-4.
- Martin, Ann (1996). Karen's Half Birthday. New York: Scholastic. ISBN 0-590-69186-4.
- Graham, Bob (2005). Oscar's Half Birthday. Cambridge: Candlewick Press. ISBN 0-7636-2699-6.