A birthday is a day that comes once a year when a person celebrates the anniversary of his or her birth. Birthdays are celebrated in numerous cultures, often with a gift, party, or rite of passage.
Many religions celebrate the birth of their founders with special holidays (e.g., Buddha's Birthday, Christmas).
Note the distinction between birthday and birthdate: The former occurs each year (e.g. April 12), while the latter is the exact date a person was born (e.g. April 19, 1985).