Why is it called a birthday?

Why is it called a birthday? Because, in China, the lunar calendar era is described by heavenly stems and earthly branches. People are born with a year, a month, a day and an hour. In the time of birth, there are two words, one is heavenly stem, the other is earthly branch, for example, in 2008, it was Wuzi. Month also has two characters, such as June 2008 10, which is the same as Guihai Day and Ugly Day. If so, the year, month, day and hour are eight characters. These eight words are determined by the time of birth. So it is called "birthday"