A birthday is an occasion when a person or institution celebrates the anniversary of their birth. Birthdays are celebrated in numerous cultures, often with gifts, cards, a party, or a rite of passage.

In most legal systems, one becomes designated as an adult on a particular birthday (usually between 12 and 21), and reaching age-specific milestones confers particular rights and responsibilities. At certain ages, one may become eligible to leave full-time education, become subject to military conscription or to enlist in the military, to consent to sexual intercourse, to marry, to marry without parental consent, to vote, to run for elected office, to legally purchase (or consume) alcohol and tobacco products, to purchase lottery tickets, or to obtain a driver's licence. The age of majority is the age when minors cease to legally be considered children and assume control over their persons, actions, and decisions, thereby terminating the legal control and legal responsibilities of their parents or guardians over and for them. Most countries set the age of majority between 18 and 21.

In many parts of the world an individual's birthday is celebrated by a party where a specially made cake, usually decorated with lettering and the person's age, is presented. The cake is traditionally studded with the same number of lit candles as the age of the individual, or a number candle representing their age. The celebrated individual will usually make a silent wish and attempt to blow out the candles in one breath; if successful, a tradition holds that the wish will be granted. In many cultures, the wish must be kept secret or it won't "come true". Presents are bestowed on the individual by the guests appropriate to her/his age. Other birthday activities may include entertainment (sometimes by a hired professional, i.e. a clown, magician, or musician), and a special toast or speech by the birthday celebrant.

The birthday cake is traditionally highly decorated, and typically covered with lit candles when presented, the number of candles signifying the age of the celebrant. The person whose birthday it is may make a silent wish and then blow out the candles. It is also common for the person celebrating their birthday to cut the initial piece of the cake as a newlywed couple might with a wedding cake. The birthday boy/girl traditionally gets to eat the first piece of the cake.

In addition to parties, it is common for people to receive gifts on their birthday. Popular gifts include toys, books, jewellery, clothes, flowers, technical devices, gift cards, checks, paper money, etc. Items such as underwear and socks are generally not as well appreciated by younger children, even if they are emblazoned with popular characters. Sometimes it is expected of the person celebrating their birthday to treat their party guests instead; this varies depending on the local culture and may involve party gifts or other gestures, for example inviting the guests to the restaurant or bar (some of them offer special birthday programs), arranging parties at home, or at work. In South Africa, however, birthdays are highly celebrated. In a given town or township, everyone is invited, including the older family members; it does not matter how young or old the person is. These celebrations are noted by big white tents being erected in the yards, which symbolize a huge celebration of some sort. Birthday presents range from gifts such as toys to animal sacrifices.

Soft drinks are often had alongside water and both sweet and savory foods are typically served to the guests. In many cultures, a birthday cake is served. Birthday parties for children often feature entertainment, costumes, party games, and a theme. A birthday party usually includes gifts for the person whose birthday it is. Most people who come to the birthday party are the ones who have the gifts.