The story goes that while on a pilgrimage to Jerusalem, it was found that a temple to Aphrodite had been built at the location of the sepulcher where Jesus was buried. Tearing down the pagan temple they found the tomb, which held three crosses within, and when contact with one healed a woman it was understood to be the true Cross.
The site would soon become the Church of the Holy Sepulcher which was dedicated on the 13th of September. The Cross was brought forth on the 14th for the church to see.
A piece of the The Cross (or Rood, in Old English) was recovered from the Persians in the year 628 and this forms another part of the celebration.
Stories of the True Cross or Holy Rood would play a big part in Christian culture through the middle ages as legends from Crusades spoke of the Rood among other religious objects such as the Holy Grail and the Spear of Destiny.