A half birthday is celebrated six months before or after the anniversary of someone's birth. For example, a June 5th birthday becomes a December 5th half birthday. For a more precise calculation, add or subtract exactly half the days in a year to a birth date.
Celebrations of half birthdays are popular among young children, multiple siblings or individuals with birthdays on major holidays. Some parents celebrate half birthdays for multiple siblings to spread apart celebrations. For parents of twins, some choose to split up birthday celebrations to emphasize individuality. Teachers may celebrate half birthdays for children with birthdays not celebrated during the school year.