Answer:
The ceremony you are referring to is likely the Osiris Festival, which was held over a period of nearly 2,000 years beginning around 2500 BCE in Abydos to honor the Egyptian god Osiris.
Some key details:
• Osiris was the god of the afterlife and the underworld in Egyptian mythology. He was regarded as the ruler of the dead and the first mummy.
• The Osiris Festival centered around reenacting the mythology of Osiris - his death at the hands of his brother Set, his resurrection by his wife Isis, and his role as lord of the underworld.
• The festival was held annually in the city of Abydos, which was closely associated with Osiris. Abydos contained numerous temples dedicated to Osiris and was an important site of pilgrimage.
• The festival likely featured prayers, songs, rituals, processions, reenactments and offerings to honor Osiris and ensure the fertility and bounty of the Nile. It drew pilgrims from all over Egypt.
• The Osiris Festival was celebrated over a very long period spanning the Old, Middle and New Kingdoms of ancient Egypt. This attests to the enduring importance of Osiris as a deity and the stability of Egyptian religious beliefs.
So in all likelihood, the ceremony you refer to that was performed for nearly 2,000 years beginning around 2500 BCE in Abydos was the Osiris Festival, held to honor the Egyptian god Osiris and reenact the rituals associated with his mythology.