The Finding of the Holy Cross
After the victory gained by Constantine by virtue of the Cross which appeared to him in the skies, and whose sign he reproduced in the Labarum, St. Helena, his mother, went to Jerusalem to try to find the true Cross. At the beginning of the second century, Hadrian had covered Calvary and the Holy Sepulchre under a terrace of 300 feet in length, on which had been erected a statue of Jupiter and a temple of Venus. The Empress razed them to the ground, and, in digging up the soil, they discovered the nails (Alleluia) and the glorious trophy to which we owe "life, salvation and resurrection" (Introit). The miraculous cure of a woman authenticated the sacred tree (Collect).
St. Helen divided into three the precious wood which had been "worthy to bear the King of Heaven" (Alleluia), which had merely been figured by the cross on which the brazen serpent was raised. One part was deposited in Rome in the church which on this account was called Holy Cross in Jerusalem the second in Constantinople and the third in Jerusalem. This last relic having been carried off by the Persians and recovered by Heraclius, this emperor solemnly brought it back to Jerusalem on May 3rd, 628. Covered with gold and precious stones, the Emperor suddenly felt himself held back by an invincible power. At this sight, Zacharias, bishop of Jerusalem, told him to imitate the poverty and humility of Jesus bearing His cross. Heraclius thereupon covered his shoulders with a common cloak and without further hindrance went his way. (Breviary September 14.)
Nos autem gloriari oportet in cruce Domini nostri Jesu Christi in quo est salus, vita, et resurrectio nostra per quem salvati, et liberati sumus, alleluia, alleluia. * Deus misereatur nostri, et benedicat nobis: illuminet vultum suum super nos, et misereatur nostri.
But it behoves us to glory in the Cross of our Lord Jesus Christ: in whom is our salvation, life and resurrection: by whom we are saved, and delivered, alleluia, alleluia. * May God have mercy on us and bless us: may He cause the light of His countenance to shine upon us, and may He have mercy on us.
(Galatians 6:14 and Psalm 66:2 from the Introit of Mass)
Deus, qui in praeclara salutiferae Crucis Inventione, passionis tuae miracula suscitasti: concede; ut vitalis ligni pretio, aeternae vitae suffragia consequimur.
O God, who in the glorious Finding of the Cross of salvation didst renew the wonders of Thy passion; grant us by the price of the wood of life to win the palm of eternal life.
O Crux splendidior cunctis astris, mundo Celebris, hominibus multum amabilis, sanctior universis: quae sola fuisti digna portare talentum mundi: dulce lignum, dulces clavos, dulcia ferens pondera: salva praesentem catervam in tuis hodie laudibus congregatam. Alleluia, alleluia.
O Cross, brighter than all the stars, famed throughout the world, lovely unto men, and of all things the most holy, which alone wast worthy to bear the ransom of the world: O sweet wood, O sweet nails, that bore so sweet a burden, save this congregation this day assembled in Thy praise. Alleluia, alleluia.
(Magnificat Antiphon from 1st Vespers)
Catholic Encyclopaedia on the True Cross: http://www.newadvent.org/cathen/04529a.htm