The Transfiguration, 6 August

The Transfiguration falls on the sixth of August, the date this rule records, commemorating the Gospel account of Christ's appearance transfigured in radiant light before the apostles Peter, James and John on a mountain, traditionally identified as Mount Tabor. The feast's antiquity differs sharply between East and West. The Eastern churches have kept it since late antiquity as one of the twelve Great Feasts of the liturgical year, among the oldest and most solemn dates on the Byzantine calendar, while the Western church kept the observance only locally and inconsistently for centuries before establishing it for the universal calendar considerably later.

That universal establishment is credited to Pope Callixtus the Third in fourteen fifty six, and the traditional account ties the timing directly to an unrelated historical event, news reaching Rome around this date of the successful defense of Belgrade against an Ottoman siege earlier that summer, a victory widely seen at the time as having halted an advance that threatened to reach much further into Christian Europe. The pope is said to have fixed the already existing but minor Transfiguration feast to this date specifically to give thanks for that victory, attaching a fifteenth century military relief to a Gospel event with no historical connection to it, one of the clearer examples on this calendar of a fixed date's meaning being reinforced, rather than originally created, by an entirely unrelated later event.