The feast of the Exaltation of the Cross sprang into existence at Rome at the end of the seventh century. Allusion is made to it during the pontificate of Sergius I (687-701) but, as Dom Bäumer observes, the very terms of the text (Lib. Pontif., I, 374, 378) show that the feast already existed. It is, then, inexact, as has often been pointed out, to attribute the introduction of it to this pope. The Gallican churches, which, at the period here referred to, do not yet know of this feast of the 14th September, have another on the 3rd of May of the same signification. It seems to have been introduced there in the seventh century, for ancient Gallican documents, such as the Lectionary of Luxeuil, do not mention it; Gregory of Tours also seems to ignore it. According to Mgr. Duchesne, the date seems to have been borrowed from the legend of the Finding of the Holy Cross (Lib. Pontif., I, p. cviii). Later, when the Gallican and Roman Liturgies were combined, a distinct character was given to each feast, so as to avoid sacrificing either. The 3rd of May was called the feast of the Invention of the Cross, and it commemorated in a special manner Saint 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 circumstances in which Heraclius recovered from the Persians the True Cross, which they had carried off. Nevertheless, 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.
(Taken from Catholic Encyclopedia)