From CatholicCulture. org:
This feast was observed in Rome before the end of the seventh century. It commemorates the recovery of the Holy Cross, which had been placed on Mt. Calvary by St. Helena and preserved in Jerusalem, but then had fallen into the hands of Chosroas, King of the Persians. The precious relic was recovered and returned to Jerusalem by Emperor Heralius in 629.
The lessons from the Breviary tell us that Emperor Heraclius carried the Cross back to Jerusalem on his shoulders. He was clothed with costly garments and with ornaments of precious stones. But at the entrance to Mt. Calvary a strange incident occurred. Try as hard as he would, he could not go forward. Zacharias, the Bishop of Jerusalem, then said to the astonished monarch: "Consider, O Emperor, that with these triumphal ornaments you are far from resembling Jesus carrying His Cross." The Emperor then put on a penitential garb and continued the journey.
Listen to more HERE about the feast.