Celebrated: in the Christian world since about the fourth century AD/CE
First, listen to T.S. Eliot read his own poem "The Journey of the Magi":
I've seen people complaining online about Christmas lights left up this
week. I wish people would leave Christmas lights up even longer, but at a
minimum, they ought to stay up until today.
On this day, twelve days after Christmas, Christians celebrate the visit of three Gentile wizards (the Magi, the Wise Men, certainly astrologers, possibly magicians) to the stable where Jesus was born. The twelve-day gap between Christmas and Epiphany is almost arbitrary, as indeed the calendar date of Christmas itself is. (This entry in the Catholic Encyclopedia refers to an early debate among church leaders over the actual time of year of Jesus' birth, which scholars pegged as May, not December.) The importance of Epiphany to Catholicism and the other Christian religions is that the announcement of the Messiah's arrival was not limited to Jews; a star in the sky, visible to all, drew these pagan priests to see the child whose birth would save humanity.
I find Epiphany one of the most moving of all Christian feasts, not only because it is all about gift-giving and generosity — the Almighty's generosity to humanity, the Wise Men's extravagant gifts of gold, frankincense and myrrh — but because of what it must have meant to Mary and Joseph, to have strangers show up and affirm their belief. The angel of the Lord appeared unto Mary, and her whole life after that must have been terrifying. She was young, she was pregnant in a way she didn't understand at all, and the man who had promised to marry her had every right to send her away. Joseph, operating on nothing but love and faith, had taken Mary and her unborn baby on an 80-mile trip from Nazareth to Bethlehem so they could be registered together as a family. All that love, all that hope, all that faith overriding all that fear and shame and anger — and here come the exotically-garbed Wise Men to say, "We saw a star. We have come to worship the King of the Jews."
What profound relief Mary and Joseph must have felt, what wonder and joy, to have their faith confirmed in such an odd and unexpected way. Luke tells us only, "Mary treasured up all these things and pondered them in her heart."