The Twelfth Day of Christmas (aka Three Kings Day, aka El Día de Reyes Magos, aka The Solemnity of the Epiphany)

January 5, Solemnity of the Epiphany

Epiphany is a large celebration, especially in Spanish speaking countries. Things look different around the household: the infant Jesus in the manger now has a small gold crown and is wearing regal robes. The figures of the wise men have reached Bethlehem, completing the nativity scene.
The Church extends itself on Epiphany to the homes of the faithful. The custom of blessing the home on this day probably originated from these words in the Gospel, "And entering into the house, they found the Child with Mary, His Mother, and falling down they adored Him." The priest blesses the house if he can be present, but if not, the father of the family may do so.

"Epiphany" "takes its name from the Greek epiphania, which denotes the visit of a god to earth. The first idea of the feast is the manifestation of Christ as the Son of God. 'Begotten before the daystar and before all ages, &nbsp the Lord our Savior is this day made manifest to the world.'" Read on that link for the second and third things also celebrated this same day.
Three Kings Day is a very big deal in Puerto Rico, Mexico, Spain and other Spanish speaking, Catholic-practicing communities, including Chile, Mexico, Honduras, Ecuador, Cuba, El Salvador, and Dominican Republic. And even here in the U.S. Great traditions, things I'd never heard of, like "Rosca de Reyes" and boxes of grass for the kings' camels!
Although I'm not Hispanic, I remember as a child, we would have our Nativity creche all set up before Christmas, sans the Infant Jesus and the Three Wise Men statues. They were kept out of sight until Christmas and The Feast of the Epiphany, respectively. I kept up that tradition raising my own son. I hope he will too someday, if ever he has his own children.
And when my son was younger and an active member of our Church's youth group, the priest directing the group began a Three Kings Day celebration where the kids and parents would all round-robin to various homes hosting different courses of a meal: appetizers, main buffet, then dessert. This was to approximate the wandering and searching of the Three Kings. At each home, we'd sing a verse of "We Three Kings," then at the final one, have a free-for-all Yankee swap (think 30-50 people and wrapped gifts numbering the same). It was some of the best fun I have ever had.
Why is Three Kings Day such a big deal?

In the West Christ's birthday had already been celebrated for some time on December 25. But with this feast was associated all the poverty and helplessness of the cave of Bethlehem; Mary and Joseph watched beside the crib in poverty, and the shepherds that came to offer their humble worship were equally poor. This aspect was lost sight of in the Feast of the Epiphany. It is true that the Magi found a poor, weak child, attended by poor parents. But through their faith they recognized and acknowledged the helpless Infant as the Redeemer and King of the world, and as such they adored Him. In the Feast of Christmas Christ is shown as man to a few of His chosen souls; in the Feast of the Epiphany, on the other hand, He appears to the whole world as God.
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The Epiphany is the high point of Christmas and the fulfillment of Advent...If the Feast of Epiphany is to be fully understood as the Church sees it, it will have to be viewed from two aspects: that of God who manifested Himself to man, and that of man, typified in the Magi, who responded with wholehearted faith and love. It is, therefore, a day of faith and grace on which no other prayer ought to take precedence over that petition of the Our Father, "Thy Kingdom Come!" — Excerpted from With Christ Through the Year, Rev. Bernard Strasser, O.S.B.

In this twelve-part series, I'd originally begun to reference the long-believed idea that the song "The Twelve Days of Christmas" was full of hidden catechism lessons. Then I read this article, which in truth is more a secular "About.com" website than a "Catholic" one, so I don't consider it a true Catholic reference source. Nonetheless, it does refocus on all the very same Catholic feast days, saints and beliefs that I've just outlined this past 12 days. No matter what version of that song's explanation you believe, it can't hurt learning that Catholics, too, have a "culture," especially a Christmas culture and that it doesn't end with Christmas Day, but rather starts with it. Because it we're not careful here in the U.S., we could lose that.