The feast of the Exaltation of the Holy Cross celebrates two historical events: the discovery of the True Cross by Saint Helena, the mother of the Emperor Constantine, in 320 under the temple of Venus in Jerusalem, and the dedication in 335 of the basilica and shrine built on Calvary by Constantine, which mark the site of the Crucifixion.
The basilica, named the Martyrium, and the shrine, named the Calvarium, were destroyed by the Persians in 614. The Church of the Holy sepulcher which now stands on the site was built by the crusaders in 1149.
However the feast, more than anything else, is a celebration and commemoration of God's greatest work: his salvific death on the Cross and His Resurrection, through which death was defeated and the doors to Heaven opened.
The entrance antiphon for the Feast of the Exaltation of the Holy Cross is: "We should glory in the cross of our Lord Jesus Christ, for he is our salvation, our life and our resurrection: through him we are saved and made free."