The Feast itself came about because of certain historical events. After the voluntary suffering and death on the Cross of the Lord, the sacred place of His suffering was scorned by the pagans. When the Roman Emperor Titus, in 70 A.D. conquered Jerusalem, he destroyed the city and leveled the Temple of Solomon on Mt. Moriah, not leaving even a stone upon a stone, as had been foretold by the Savior in a dialogue with His disciples [Mark 13:1-2].

The Emperor Hadrian (117-138), a backward, zealous pagan, constructed in place of the Jerusalem destroyed by Titus a new city, which he named Helio-Hadrianopolis. Further, it was forbidden for this city to be called by its previous name of Jerusalem. He commanded that the Holy Grave of the Lord be covered with earth and stones, raising on it an idol. On Golgotha, where the Savior was crucified, in 119AD he erected a temple dedicated to the goddess Venus. Sacrifices were offered before the statue and pagan rites were celebrated, accompanied by prostitution. In Bethlehem, in the place where the Savior had been born of the Most-Pure Virgin, the impious monarch erected an idol to Adonis. All of this he did intending that the people completely forget about Christ the Savior and nevermore recollect the place where He lived, taught, suffered and arose with glory.

When Emperor Constantine the Great, Equal-to-the-Apostles (306-337) ascended the throne (being the first of the Roman Emperors to recognize Christianity), he, together with his pious mother, Augusta Helen, decided to restore the city of Jerusalem, and in the place of the suffering and resurrection of the Lord Jesus Christ to erect a new church, to cleanse all of the places connected with the memory of Jesus from the pagan cult, and again to consecrate all of them. The Orthodox Queen Helen left for Jerusalem with a great quantity of gold, and the Emperor sent a letter to Patriarch of Jerusalem Macarius 1st (313-323) in which he asked every kind of aid in the holy task of restoring the Christian holy places.

Having arrived in Jerusalem, the pious Queen destroyed all the idols and cleansed the city of pagan cult objects, consecrating the defiled places. She burned with the desire to raise up the Cross of Our Lord Jesus Christ; and so she commanded that digging proceed at the place where the Temple of Venus had stood. Finally, in 325, the covered Grave of the Lord was discovered, as well as the place of execution, not far from which were found three crosses and four nails, as well as the sign board which had been nailed over His head.

In order to determine which of the three crosses belonged to the Savior, Patriarch Macarius ordered that the crosses, in turn, be placed on a dead person who was being brought to a place of burial. When the Cross of Christ touched the dead one, he immediately came to life. With great joy, the Orthodox Queen and the Patriarch together lifted up the Life-Creating Cross and showed it to all the people standing by. Later the Church of the Holy Sepulchre was constructed on the site, enclosing within its walls the place of the crucifixion of the Savior, as well as His tomb, and a Feast was instituted for September 14, commemorating the glorious Exaltation of the Cross.

Extracted from the site:
http://www.orthodoxworld.ru/english/feasts/2/index.htm