Birthday

A birthday is the date on which a person was born. It is customary to celebrate the anniversary of one's birthday in some way, usually by having a birthday party.

The traditional song to sing to someone on their birthday is "Happy Birthday".

People often get gifts on their birthday.

Astrological sun signs roughly correspond with birthdays.

Notable birthdays can include:

When the most significant digit changes, for example one's 1st, 10th, 20th, 50th, or 100th birthdays.
Jewish girls have a bat mitzvah on their 12th birthday; Jewish boys have a bar mitzvah on their 13th.
Many girls have "sweet sixteen" birthday parties.
In the United States, one is legally an adult on one's 18th birhday.
In the U.S., the legal drinking age is 21; on your 21st birthday the custom is for your older friends to drag you to the bars and get you rip-roaring drunk.

Sometimes the birthdays of historically significant people, like Jesus or Martin Luther King, Jr., get turned into holidays.