 In the book "El Calendario Mexica y la Cronografia" by Rafael Tena and published by the National Institute of Anthropology and History of Mexico, the author gives the last day of the Nahuatl month Panquetzaliztli as the date of the celebration of the rebirth of the Lord Huitzilopochtli on top of Coatepec (Snake Hill); December 9 in the Julian calendar or December 19 in the Gregorian calendar with the variant of December 18 in leap years.The most important and powerful structure in Tenochtitlan is the Templo Mayor.Its importance as the sacred center is reflected in the fact that it was enlarged frontally eleven times during the two hundred years of its existence.