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.
Western private birthday traditions
History of celebration of birthdays in the West
It is thought that 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. Before this, such celebrations were not common; and, hence, 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.
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.
Even today, the celebration of birthdays is not universal in the West; in addition to those people preferring name day celebrations, 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. And in some activities that are not year-round, birthday acknowledgements may be discouraged because of some birthdays not falling during the season.
In most English-speaking countries it is traditional to sing the song Happy Birthday to You to the honored person celebrating his birthday. The Happy Birthday Song melody is thought to be the most frequently sung melody in the world. Similar songs exist in other languages such as "Lang zal hij/zij leven" (and several others) in Dutch, "Zum Geburtstag Viel Glück" in German, "Cumpleaños feliz" in Spanish, "Sto lat" in Polish, "Joyeux Anniversaire" in French, and "Tanti Auguri a te" in Italian. This happens traditionally at a birthday party while someone brings a birthday cake into the (often darkened) room.
The birthday cake is traditionally highly decorated, and typically covered with lit candles when presented; the number of candles often equals the age of the person. The person whose birthday it is makes a silent wish and then blows out the candles; if done in one breath, the wish is supposed to come true, but only if the person keeps the wish to himself (or herself). It is also common for the "birthday boy" or "birthday girl" to cut the initial piece of the cake as a newlywed couple might with a wedding cake. Birthday cakes have been a tradition dating back as far as the Middle Ages when the English would conceal symbolic items such as gold coins, rings and thimbles inside their cakes. Each item was associated with a prediction. For example, a person finding a gold coin in a birthday cake would supposedly become wealthy where a person discovering a thimble would never marry.
Typical birthday party decorations include balloons, streamers, and confetti.
For special birthdays and for when the number of candles might be considered impractical or a fire hazard, special candles might be substituted for the many individual candles. These candles are in the shape of a numeral; for example, on the fifth birthday there may be one candle on the cake in the shape of the numeral 5, and on the fiftieth birthday there may be two candles on the cake, one in the shape of the numeral 5 followed by the other in the shape of the numeral 0.
A birthday is considered a special day for the person, and so the person will often get special treatment from friends and family. This is especially true for children who cannot wait for their own special day. In addition to parties, people often get gifts on their birthday.
Birthday parties for children often include fun games which are relevant to the local culture.
There are also traditions of surprise parties.
Not all traditions are equally generous. In certain circles the birthday boy or girl is expected to treat their party guests, this varies depending on the local culture and may involve party gifts or a nice gesture. In some cultures the birthday that means reaching the legal age limit for drinking alcohol may be celebrated with a party with free or abundant alcoholic drinks.
Notable birthdays can include:
When the most significant digit changes, for example one's 1st, 10th, 15th, 30th, 50th, or 100th birthdays, as well as significant years such as the 18th (US legal adult age), and 21st (US alcoholic beverage drinking age).
One's golden birthday, also called a champagne birthday, is the day when the age someone turns is the same as the day in the month he or she was born. (for example, someone turning 26 on December 26 celebrates his or her golden birthday).
In most legal systems, one becomes a legal adult on a particular birthday, and at different ages gain different rights and responsibilities — voting, certain drug use (for example, alcohol, purchasing tobacco), eligibility for military draft or voluntary enlistment, purchasing lottery tickets, etc.
Most cultures have one or more coming of age birthdays:
Jewish boys have a bar mitzvah on or around their 13th birthday. Jewish girls observe a bat mitzvah on or around their 12th birthday, or sometimes on or around their 13th birthday in Reform and Conservative Judaism.
In some Christian traditions, generally Catholic and Anglican, Confirmation is the ritual by which a young person becomes an official member of the Church. This sometimes includes the bestowal of a 'Confirmation name,' generally the name of a saint, which is often worn as a second middle name. Confirmation also bestows the Holy Spirit upon the confirmand, and in some churches is received concurrent with baptism or first communion.
In Latin America the quinceañera celebration traditionally marks a girl's 15th birthday.
Some girls and a few boys in the United States have "sweet sixteen" birthday parties.
In many Asian countries the 14th birthday is celebrated as the day one becomes a man, or a woman, in society.
The birthdays of historically significant people, like national heroes or founders, are often commemorated by an official holiday. Some saints are remembered by a liturgical feast (sometimes on a presumed birthday). By analogy, the Latin term Dies natalis 'birthday' is applied to the anniversary of an institution (such as a university).
Official/Alternative birthdays and name day
Some notables, particularly Monarchs, have on a fixed day of the year an official birthday which do not match their actual birthday, but on which celebrations are held. Examples are:
King's or Queen's Official Birthday in Australia, Canada, Fiji, New Zealand, and the United Kingdom.
Commonwealth Day, originally called Empire's Day, is on the Queen-Empress Victoria's birthday: May 24
Jesus of Nazareth's official birthday is celebrated as Christmas Day around the world: 25 December or 7 January in Julian calendar interpretations.
The Grand Duke's Official Birthday in Luxembourg: 23 June
King's official birthday in Belgium: November 15 (on saint Leopold, liturgical feast of the dynasty's founder's patron saint)
in Malaysia, the Yang di-Pertuan Agong (federal King, elected for 5 years) on 3 June
Koningsdag or Koninginnedag in the Kingdom of the Netherlands is fixed on 30 April (Queen's Day; celebration of the reigning Queen's accession). It was fixed by queen Beatrix at the birthday of former queen Juliana, to avoid the winter weather associated with her own birthday in January.
While it is uncommon to have an official holiday for a republican head of state's birthday, this can become a permanent posthumous honour, especially in the case of a so-called father of the fatherland, for example George Washington (best known as Presidents' Day; also celebrated in the US is Lincoln's Birthday)
In cases where a mythical figure's actual birthday is unknown, it is common for a particular date to be substituted.
People who are born on the leap day 29 February, which only occurs during leap years, often celebrate their birthday in other years on the 28th (the last day of February), or 1 March (the first day they have, measured in whole years, a new age).
In some Roman Catholic and Eastern Orthodox countries such as France, Hungary, or Greece, it is common also to have a 'name day'/'Saint's day'. This is celebrated in much the same way as a birthday, but is held on the official day of a saint with the same Christian name as the birthday boy/girl; the difference being that one may look up a person's nameday in a calendar, or easily remember common namedays (for example, John or Mary); however in pious traditions, the two were often made to concur by giving a newborn the name of a saint celebrated on its birthday, or even the name of a feast, for example, Noel or Pascal (French for Christmas and "of Easter"). In some countries, name days are celebrated with much more elaborate festivities than birthdays; in the past, birthdays often weren't celebrated at all in those countries.
In school, a half-birthday or other unbirthday is sometimes celebrated for those whose birthdays do not fall on a school day (especially for birthdays falling during holiday and vacation periods).
All racehorses traditionally celebrate their birthday on (that is, calculate their age in years from) 1 August in the Southern Hemisphere, and on 1 January in the Northern Hemisphere.
Birthday gift symbolism
A birthstone is a gift of a precious material (jewelry, mainly gemstones; themselves traditionally associated with various qualities) that symbolizes the month of birth (in the Gregorian Calendar).
It is sometimes also called birthday stone (cf. infra; but that word is, confusingly, sometimes used as a synonym for an anniversary gift, which is related to the recipient's age, that is, year of birth).
There have been many different sets of birthstones used throughout history and in different cultures. In 1912, in an effort to standardize them, the American national association of jewelers, Jewelers of America, officially adopted the following list; it is currently the most widely used list in the United States and many other locations, including Australia and Thailand. Some alternates have been adopted to be a less expensive substitute for a cut stone.
Month Birthstone Alternate
January Garnet Rose Quartz
February Amethyst Onyx
March Aquamarine Red Jasper (Bloodstone)
April Diamond Rock Crystal (Quartz)
May Emerald Chrysoprase
June Pearl (the only non-mineral) Alexandrite or Moonstone
July Ruby Jade or Carnelian
August Peridot Aventurine, Sardonyx, or Sapphire
September Sapphire Lapis lazuli
October Opal Pink Tourmaline
November Yellow Topaz Citrine or Turquoise
December Blue Zircon Turquoise, Lapis Lazuli or Tanzanite
Tanzanite was added to December by the American Gem Trade Association in 2002.
The birthstone seems to originate from Biblical times. The Breastplate of Aaron, referred to in Exodus 39:10-14:
10 Then they mounted four rows of precious stones on it. In the first row there was a ruby, a topaz and a beryl;
11 in the second row a turquoise, a sapphire and an emerald;
12 in the third row a jacinth, an agate and an amethyst;
13 in the fourth row a chrysolite, an onyx and a jasper. They were mounted in gold filigree settings.
14 There were twelve stones, one for each of the names of the sons of Israel, each engraved like a seal with the name of one of the twelve tribes.
The precise list of birthstones however can be found in Revelation 21:19-20 where the foundation stones of the new Jerusalem are listed, in the order of the Roman calendar:
19 And the foundations of the wall of the city were garnished with all manner of precious stones. The first foundation was jasper; the second, sapphire; the third, a chalcedony; the fourth, an emerald;
20 The fifth, sardonyx; the sixth, sardius; the seventh, chrysolyte; the eighth, beryl; the ninth, a topaz; the tenth, a chrysoprasus; the eleventh, a jacinth; the twelfth, an amethyst.
Zodiac stones and astrology
An astrological version of the birthstones exists using the twelve zodiac signs instead of the twelve calendar months. Listed below are the dates for the tropical sun signs and their associated birthstone.
Aries (March 21 - April 19) Diamond
Taurus (April 20 - May 20) Emerald
Gemini (May 21 - June 20) Crystal
Cancer (June 21 - July 22) Pearl
Leo (July 23 - August 22) Ruby
Virgo (August 23 - September 22) Sapphire
Libra (September 23 - October 23) Opal
Scorpio (October 24 - November 21) Zircon
Sagittarius (November 22 - December 21) Turquoise
Capricorn (December 22 - January 21) Garnet
Aquarius (January 22 - February 18) Amethyst and moonstone
Pisces (February 19 - March 20) Aquamarine
March Daffodil (Jonquil)
April Dahlia, Sweet Pea
May Sunflower, Lily of the Valley
June Honeysuckle, Rose
August Lily, Gladiolus
September Forget-me-not, Aster
October Calendula (Marigold), Rose, Camellia
December Holly, Narcissus
Just as there are alternatives with birthstones, there are also alternatives with birth flowers. For example, October is often listed as Calendula (Marigold) , but is also occasional noted as being rose or camellia.
The most common birthday in the USA is October 5, whereas the least common birthday is May 22. 
A person's birthday is usually recorded according to the time zone of the place of birth. Thus people born in Samoa at 11:30pm will record their birthdate as one day before UTC and those born in the Line Islands will record their birthdate one day after UTC. They will apparently be born two days apart, while some of the apparently older ones may be younger in hours. Those who live in different time zones from their birth often exclusively celebrate their birthdays at the local time zone.
While this word has also been used as synonym of Birth stone (see above), there is a separate list of assignment according to the day of the week of the recipient's birth:
Monday : Pearl (the only non-gem)
Tuesday : Garnet
Wednesday : Cat's eye (Chatoyant Chrysoberyl)
Thursday : Emerald
Friday : Topaz
Saturday : Sapphire
Sunday : Ruby