Exaltation of the Holy Cross
Roodmas 1 — more commonly known simply as “Holy Cross Day” — was first begun to commemorate the Dedication of the Basilica of the Resurrection, built by St. Helena (Constantine the Great’s mother), in Jerusalem in A.D. 355 — but the true Cross was found shortly thereafter, also by St. Helena, so the two events were joined. The story of the finding of the True Cross, from the Catholic Encyclopedia:
In the year 326 the mother of Constantine, Helena, then about 80 years old, having journeyed to Jerusalem, undertook to rid the Holy Sepulchre of the mound of earth heaped upon and around it, and to destroy the pagan buildings that profaned its site. Some revelations which she had received gave her confidence that she would discover the Saviour’s Tomb and His Cross.