Holy Cross Day is the name given in the Book of Common Prayer to the Feast of the Exaltation of the Cross, which is kept on September 14th. In The Rites of Durham (1593) it is still called Holy Rood Day, after the Anglo-Saxon manner.
The feast goes back to the fourth century, when Helen, dowager Empress of Constantinople, declared that the true Cross of Christ had been found. The relic was treasured up in a silver casket in the newly consecrated Church of the Holy Sepulchre in Jerusalem (AD 335), and brought out for display and veneration every year on this date. See St Helen Finds the True Cross in The Copy Book.
The prayer below is the Collect for this day according to the Sarum Use, the liturgy of the English Church prior to the Reformation in the sixteenth century.
Collect
O GOD, Who wast pleased to redeem mankind with the precious Blood of Thy Only Begotten Son, our Lord Jesus Christ, mercifully grant that they who draw nigh to adore the life-giving Cross may be set free from the bonds of their sins. Amen.