From the Treasury of Daily Prayer:

One of the earliest annual celebrations of the Church, Holy Cross Day traditionally commemorated the discovery of the original cross of Jesus on September 14, 320, in Jerusalem. The cross was found by Helena, mother of Roman Emperor Constantine the Great. In conjunction with the dedication of a basilica at the site of Jesus’ crucifixion and resurrection, the festival day was made official by order of Constantine in AD 335. A devout Christian, Helena had helped locate and authenticate many sites related to the life, ministry, death, and resurrection of Jesus throughout biblical lands. Holy Cross Day has remained popular in both Eastern and Western Christianity. Many Lutheran parishes have chosen to use “Holy Cross” as the name of their congregation.