The Sed festival (ḥb-sd, conventional pronunciation ; also known as Heb Sed or Feast of the Tail) was an ancient Egyptian ceremony that celebrated the continued rule of a pharaoh. The name is taken from the name of an Egyptian wolf god, one of whose names was Wepwawet or Sed. The less-formal feast name, the Feast of the Tail, is derived from the name of the animal's tail that typically was attached to the back of the pharaoh's garment in the early periods of Egyptian history. This tail might have been the vestige of a previous ceremonial robe made out of a complete animal skin.
The ancient festival might have been instituted to replace a ritual of murdering a pharaoh who was unable to continue to rule effectively because of age or condition. Eventually, Sed festivals were jubilees celebrated after a ruler had held the throne for thirty years and then every three to four years after that. The festival, primarily, served to reassert pharaonic authority and state ideology. Sed festivals implied elaborate temple rituals and included processions, offerings, and such acts of religious devotion as the ceremonial raising of a djed, the base or sacrum of a bovine spine, a phallic symbol representing the strength, "potency and duration of the pharaoh's rule". The festival also involved symbolic reaffirmation of the pharaoh's rulership over Upper and Lower Egypt. Pharaohs who followed the typical tradition, but did not reign so long as 30 years had to be content with promises of "millions of jubilees" in the afterlife.
View the full Wikipedia page for Sed festival