By Jason DeRusha, WCCO-TV

MINNEAPOLIS (WCCO) — To children of all ages, a birthday is a reason to celebrate. But why? What are the reasons for many of our birthday celebrations?

• Why do we celebrate birthdays?

The idea of celebrating the date of your birth is a pagan tradition. In fact, many Christians didn’t celebrate birthdays historically, because of that link to paganism.

Pagans thought that evil spirits lurked on days of major changes, like the day you turn a year older.

The ancient Greeks believed that each person had a spirit that attended his or her birth, and kept watch. That spirit “had a mystic relation with the God on whose birthday the individual was born,” says the book The Lore of Birthdays.

• Why do we blow out candles on our birthday?

The candles were a response to the evil spirits. They showed up to communicate with the gods. A light, in the darkness.

The Germans are credited with starting the kids birthday tradition in the 1700s. They put candles on tortes for “kinderfeste,” one for each year of life, along with some extras to signify upcoming years.

• Why do we sing “Happy Birthday To You?”

It’s the most recognizable song in the English language, according to the Guinness Book of World Records, and it started as a song for schoolkids.

In 1893, two Kentucky schoolteachers, Patty and Mildred Hill wrote “Good Morning To All.” The tune was published in a book for schoolteachers.

It’s unclear who changed the words to “Happy Birthday To You,” but in 1933, that song was in an Irving Berlin musical. One of the Hill’s sisters sued, arguing that they held the copyright to the song. They won the case, and the courts have ruled that copyright still holds today.

In fact, some believe the song is under copyright until 2030. The owner of the copyright splits proceeds with the Hill’s estate, reportedly $2 million a year.

• What’s the most common birth date?

Oct. 5 is considered to be the most common birthday in the United States. The reason is pretty obvious: go back 9 months, and you’ll find a conception date of New Year’s Eve. May 22 is considered to be the least common birthday in the United States.