The celebration on September 14 was imitated in other places, and particularly at Rome by the end of the seventh century. Relics of the cross were also brought to various places, particularly the Church of Santa Croce in Gerusalemme in Rome. In AD 615 the armies of Persia under King Chosroes II took the relic of the Cross from Jerusalem; it was recovered and restored by the Emperor Heraclius in 629.
Now the churches of Gaul at that time seem to have been unaware of the feast on September 14, but celebrated the Cross on May 3 a date that may have been derived from a legend of the finding of the Cross. In latr years the Gallican and Roman liturgies were combined, each of the two feasts was given a different character so that both could be celebrated. The 3rd of May was called the feast of the Invention [finding] of the Cross, and it paticularly commemorated St Helena's discovery of the sacred wood of the Cross; the 14th of September, the feast of the Exaltation of the Cross, commemorated above all the recovery of the the relics by Heraclius. “Nevertheless,” as the old Catholic Ecyclopaedia puts it, “it appears from the history of the two feasts, which we have just examined, that that of the 13th and 14th of September is the older, and that the commemoration of the Finding of the Cross was at first combined with it.”
A strong devotional reason for this celebration of the Holy Cross in September is given by Stephen Reynolds in For All the Saints (page 278),a nd is worth quoting here:
Jesus was crucified at the time of year when people in the northern hemisphere prepare the earth for planting. But September is harvest, our time for reaping and sharing what the earth, under our care, has brought forth. Just so with our remembrance of the Holy Cross. On Good Friday we recalled its planting in the seedtime of the new creation; and now, on the verge of autumn, we look for Christ, the true Vine which the Cross supported, to bear the fruit of justice and mercy not only in opur own lives but also in the dealings of the world.
For the Legend of the True Cross, see the homily for Good Friday, which is posted at my sermon blog, Sermonets for Christianets, on June 16 of this year. There should be a link over on the left somewhere.has succeeded in showing that, in spite of what various Protestant or Rationalistic authors have pretended, the fragments of the Cross brought together again would not only not "be comparable in bulk to a battleship", but would not reach one-third that of a cross which has been supposed to have been three or four metres in height, with transverse branch of two metres (see above; under I), proportions not at all abnormal (op. cit., 97-179). Here is the calculation of this savant: Supposing the Cross to have been of pine-wood, as is believed by the savants who have made a special study of the subject, and giving it a weight of about seventy-five kilograms, we find that the volume of this cross was 178,000,000 cubic millimetres. Now the total known volume of the True Cross, according to the finding of M. Rohault de Fleury, amounts to above 4,000 000 cubic millimetres, allowing the missing part to be as big as we will, the lost parts or the parts the existence of which has been overlooked, we still find ourselves far short of 178,000,000 cubic millimetres, which should make up the True Cross.”