In honor of Wil Wheaton's birthday (as well as my own) I thought I would look up how come we celebrate birthdays to begin with.
Most people today who celebrate birthdays tend to have parties and the birthday person receives gifts from them, along with some rendition of a birthday song as they blow out an ever-increasing number of candles on their cakes. Even people who don't go that far still get a lot of "happy birthday" wishes throughout the day. It's become a tradition of sorts.
Birthdays started to be celebrated as soon as calendars were invented. In early civlizations where they used horoscopes of ruling monarchs, they wanted the most exact date possible to ensure a good casting. Birthday omens were meticulously examined because the prospects of the ruler directly affected the prospects of his subjects.
But the exact origin of birthday celebrations is lost somewhere in the past, even before these events. What we do know is that it is human nature to desire personal attention and also to celebrate, so the eventuality of the birthday celebration seems inevitable.
Birthdays most probably occurred when the way time was kept was altered. In ancient times people kept track of time through the changing movements of the sun or the moon, or by some important event that happened at regular intervals such as a passing comet. But the calenders were formulated in respect to lunar or solar positions, making it possible to mark time changes and other special days.
Most people believe that birthday celebration began with the pagan people, who had a great fear of occult powers and held them responsible for calamities. They thought people were susceptible to evil spirits and the birthdate added one more year to their life, changing the numbers that would affect them. They also believed that surrounding the person celebrating their birthday should be surrounded with laughter and joy to shield him or her from harm or evil spirits.
Historically, however, the only early evidence of celebrating births seems to occur among the nobility (an event which recently occurred for Prince George last week). Later on, the Germans are credited with celebrating all birthdays, especially those of children. They called such days "kinderfeste", or "Child's party". Over time the tradition found its way into Christianity even though at first the idea had seemed abhorrent, and now it seems like almost everywhere in the world the tradition persists.
The most usual song at these parties today is "Happy Birthday to You", which was written in 1893, technically, but altered afterwards as this article explains in more detail.