The Feast of the Exaltation of the Holy Cross, or the Triumph of the Cross, is a feast that celebrates the finding of the True Cross, the dedication of churches built on the site of the Holy Sepulchre and Mount Calvary and the restoration of the True Cross to Jerusalem in 629.
The True Cross was discovered in 326 by Saint Helena during a pilgrimage of hers to Jerusalem and the Church of the Holy Sepulchre was built on that site by order of both Saint Helena and her son, Saint Constantine the Great with the dedication coming only 9 years later. Though the dedication of the church originally occurred on September 13th, the following day the Cross was brought outside of the church so that the faithful would be able to pray before it and venerate it.
Until 1969, the Wednesday, Friday and Saturday of the calendar week following the Feast were known as Ember days, though this designation is used only in the Roman Rite's Extraordinary form, Anglican Ordinariate and the Western Orthodoxy. The Rule of Saint Benedict prescribes this feast day as the beginning of Monastic Winter 0 a period with three nocturns of psalms and readings at matins). The liturgical color of the day is red and the readings in the Mass of Paul VI draw a comparison between the bronze serpent found in Numbers and John 3:14-15 --
"And just as Moses lifted up the serpent in the desert, so must the Son of Man be lifted up, so that everyone who believes in him may have eternal life."