Thaborium (Θαβώριον or Μεταμόρφωσις, Festum Tranfigurationis, s. Patefactionis Christi), the Feast of the Transfiguration of Christ. It was exalted to a feast of universal observance by pope Calixtus III in 1457, the day assigned to it being August 6. The ancient Church had not altogether ignored, but none the less greatly neglected, its observance. The purpose of its modern revival was the commemoration, first, of the transfiguration of Christ, and, second, of the defeat of the Turks at the siege of Belgrade in 1456. See Augusti, Christl. Archäologie (Leips. 1820), 3, 292 sq.; Herzog, Real-Encyklop. s.v.