What is the origin of the feast of Epiphany?

In his book Spirit of the Liturgy, the future Pope Benedict XVI wrote,

It is hard to say how far back the beginnings of the Christmas feast go. It assumed its definitive form in the third century.

At about the same time the feast of the Epiphany emerged in the East on January 6 and the feast of Christmas in the West on December 25.

The two feasts had different emphases because of the different religious and cultural contexts in which they arose, but essentially their meaning was the same: the celebration of the birth of Christ as the dawning of the new light, the true sun, of history.