December 12 is the feast day of Our Lady of Guadalupe, Patroness of the Americas. It recalls Mary's apparitions at Tepeyac, Mexico, to St. Juan Diego, an Aztec convert.
When Our Lady appeared on December 9, 1531, she asked Juan Diego to go to the Bishop and request that a church be built on the hill of Tepeyac, now part of Mexico City. The Bishop asked a sign from the Lady. The sign was given on December 12, when Mary again appeared to Juan and told him to pick the Castilian roses (native to the Bishop’s homeland) which miraculously appeared. Juan Diego gathered them into his cloak (tilma) and brought them to the bishop. When he opened the tilma, the Image of Our Lady was revealed on it. Our Lady’s request was fulfilled. The event brought an end to the Aztec practice of human sacrifice, the restoration of peace in the land, and the conversion of over ten million over the next decade. The tilma’s miraculous image is venerated in the Basilica in Mexico City.
For more information on the image, and what it would mean to the Aztecs, see the page Our Lady of Guadalupe.