A party that is held for a person exactly three months to the day after someones birthday. Because its one quarter of a year since there Birthday.

He was born on April 5th, so his quarter Birthday will be on July 5th.
Its his Quarter Birthday.
by judge dredd7 October 11, 2012
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A party held for someone nine months to the day after there birthday because it has been three quarters of a year since there birthday.

He was born on June 12th so his three quarter birthday will be on March 12th.
by judge dredd7 October 12, 2012
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