On the Feast of the Exaltation of the Cross (or Triumph of the Cross) we honor the Holy Cross by which Christ redeemed the world. The public veneration of the Cross of Christ originated in the fourth century, according to early accounts, beginning with the miraculous discovery of the cross on September 14, 326, by Saint Helen, mother of Constantine, while she was on a pilgrimage to Jerusalem -- the same day that two churches built at the site of Calvary by Constantine were dedicated.
The observance of the Feast of the Exaltation (probably from a Greek word meaning "bringing to light") of the Cross has been celebrated by Christians on September 14 ever since. In the Western Church, the feast came into prominence in the seventh century, apparently inspired by the recovery of a portion of the Cross, said to have been taken from Jerusalem the Persians, by the Roman emperor Heraclius in 629.
Christians "exalt" the Cross of Christ as the instrument of our salvation. Adoration of the Cross is, thus, adoration of Jesus Christ, the God Man, who suffered and died on this Roman instrument of torture for our redemption from sin and death. The cross represents the One Sacrifice by which Jesus, obedient even unto death, accomplished our salvation. The cross is a symbolic summary of the Passion, Crucifixion and Resurrection of Christ -- all in one image.
Real all here. [h/t: New Advent]