Birthday is the name given to the day that some people in many cultures celebrate the anniversary of the day they were born. It is often marked by a birthday party and/or friends when gifts are 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.

It is thought the large-scale celebration of birthdays in Europe began with the cult of Mithras, which originated in Persia but was spread by soldiers throughout the Roman Empire. Such celebrations were uncommon previously so practices from other contexts such as the Saturnalia were adapted for birthdays. Because many Roman soldiers took to Mithraism, it had a wide distribution and influence throughout the empire until it was supplanted by Christianity. The Jewish perspective on birthday celebrations is disputed by various rabbis.

The celebration of birthdays is not universal. Some people prefer name day celebrations, and Jehovah's Witnesses do not celebrate either, considering their origins to be pagan festivals along with Christmas and Easter. Some adults loathe celebrating it as it reminds them that they are getting progressively older.