The tradition of El Dia de los Tres Reyes Magos (literally, the Day of the Three Magi Kings)--aka the Epiphany--is a holiday that originated in Spain and is now celebrated in Hispanic cultures around the world on January 6th.
The Magi are those three Wise Men--the kings who traveled to Bethlehem to visit the infant Jesus and bring him gifts of Gold, Frankincense, and Myrrh. The holiday is meant to celebrate the appearance to the world of the baby Christ.
As part of the tradition, which marks the end of the Christmas season in Spanish-speaking countries, children write letters to the Three Magi asking for gifts, much like American children write to Santa Claus. And just as children leave carrots out for Santa's reindeer, Argentinean children leave out grass and water for the Kings' camels.
The children's little shoes are left out on the night of January 5th by the door of the house, and on the morning of January 6, they find a gift there. (Below, a picture of our Esteban's shoes filled with chocolate coins.)