The Feast of the Exaltation of the Holy Cross

Commemorates three historical events: the discovery of the True Cross in 326, its initial exaltation for public veneration in 335, and the recovery of the Cross from the Persians in 628. Following the discovery of the True Cross, Mount Calvary, and Christ’s tomb, Emperor Constantine ordered the construction of a church to encompass these sites and house the True Cross. The Church of the Holy Sepulcher was erected and dedicated on September 13, 335. As the legend goes, the following day, Christ’s Cross was brought outside the newly built church for the faithful to venerate. Thus, the first showing, or Exaltation of the Holy Cross, might have occurred on September 14, 335. 

(Source: My Catholic Life)