The Sed Festival (Royal Jubilee)
After thirty years on the throne, the king had to run a sacred course to prove he could still rule.
Also: Sed festival · Heb-Sed · Heb Sed · royal jubilee · Sed-festival · jubilee of the king · ebony label of Den
Egyptians held a great royal jubilee, the Sed festival, traditionally after a king had reigned thirty years, to renew his strength and confirm his fitness to rule. Its rites — including a ritual run by the king — re-enacted his coronation and rejuvenation. Some kings celebrated it early or repeatedly; its imagery is among the oldest in Egyptian kingship.
In short
The royal jubilee (heb-sed), a rite of renewal traditionally celebrated after thirty years of a king's reign (and repeated thereafter), in which the king's vigor and right to rule were ritually regenerated.
Where it’s found
Attested from the Early Dynastic Period (the ebony label of Den, Dynasty 1); depicted throughout Egyptian history
How the idea developed across Egypt
Among the most ancient royal rites — imagery appears already in Dynasty 0–1 (e.g. the ebony label of Den, Dynasty 1, depicting the king running the Sed-festival course) — and continues into the Ptolemaic period; the 'thirty-year' rule was an ideal not always observed (some kings, e.g. Amenhotep III and Ramesses II, celebrated multiple jubilees).
Earliest secure attestation cited here is the ebony label of Den (Dynasty 1). The thirty-year timing is an ideal, frequently varied in practice; do not state a king's jubilee count without sourcing.