"Happy Birthday to You" is a song which is sung to celebrate the anniversary of a person's birth.
Birthday celebrations were rare during the Middle Ages but saw a resurgence with the advent of the Reformation. During this period, they were seen as a good way to transfer customs from the saint's days to other dates not linked to the newly repudiated veneration of saints.
A birthday is the date on which a person is born, marking the day a life outside the womb begins. It is customary in many cultures to celebrate the anniversary of one's birthday, for example by having a birthday party with family and/or friends. Gifts are often given to the person celebrating the birthday. It is also customary to treat people specially on their birthday, either generally acceding to their wishes, or subjecting them to a rite of transition.