It is celebrated on May 7 each year.
The first appearance of the Holy Cross was in Constantine the Great, with the triumphal emblem of victory: “ἐν τουτῳ νικα”. Here we have another appearance, which took place in Jerusalem, about 346 AD. At that time the archbishop of Jerusalem was Cyril (see March 18) and the king was Constantine, son of Constantine the Great.
This apparition took place on May 7, one of the days of Pentecost. It was the (Byzantine) third hour, when suddenly the Holy and Life-giving Cross appeared in the sky, formed by a dazzling light, above Golgotha, as far as the Mount of Olives. This supernatural sight caused great admiration and emotion to all present in Jerusalem. Young and old, women and children, all together, ran to the church and with much joy and warm delight thanked and glorified God, our Lord Jesus Christ, who sanctified the wood of the cross and made this point the most powerful weapon of the struggling Christians against of the devil.