The Cross on which Our Blessed Lord suffered for us was, following Jewish usage, buried on Mt. Calvary after the crucifixion. The precise spot was revealed in a vision to the Empress Helena, mother of Constantine, the First Christian Emperor.
This date commemorates the recovery of the True Cross in the reign of the Emperor Heraclius and its delivery by him about the year 629 into the hands of Patriarch Zacharias of Jerusalem, from which city it has been carried away some years previously by the Persians with the object of transporting it to their own country.
God has been pleased to give so much power to the Cross that at its sign alone the demons fly; by it the priest blesses the faithful, the devout receive abundant graces. The early Christians had so much devotion to it that, according to the ancient Fathers, they never began any action without first making the sign of the cross upon themselves.
In the Middle Ages no public deed, inscription, law etc., was begun to be written without first tracing upon it the sign of the cross. This sign was accepted as the signature of the uneducated; it often preceded that of ecclesiastics, and in many country districts even the dough and the bread were marked with a cross before they were baked.
(Source: The New Roman Missal by Father Lasance)
Commentary: The True Cross has been divided into thousands of very small pieces and distributed throughout the Catholic world to her churches. I personally have had the opportunity to adore and kiss a piece of the True Cross.