The Djed-pillar was attested already before the first mentionings of Osiris. It is suggested that it might be a Predynastic symbol, perhaps representing a pole around which grain was tied. By time, the word 'djed' came to mean 'stability' or 'continuity of power' and its link with the King is thought to symbolize and enhance the strength and endurability of kingship. It is often seen used in decorative friezes together with the Ankh and Was sceptre hieroglyphs but just as frequently with the 'Tyet' knot, symbol of Aset.

It existed already about 2600 BC, and an early depiction of it is as a decoration on the tiles in the Step Pyramid at Saqqara.

There are various ideas about what it actually depicts but no consensus has sofar been reached. In the Book of Going Forth By Day it is called the vertebrae or backbone of Wesir/Osiris, and there are papyri showing the Djed pillar with human arms holding the crook and flail, an surrounded with the 'atef' crown.

The 'Raising of the Djed-pillar'

There is also a ritual called the 'Raising of the Djed pillar', believed to have been started by the kings at MenNefer (Memphis). It is recorded as having formed a part of a royal jubilee ritual for Senwosret I (Dyn 12), and it is also depicted in the tomb of one official named Kheruef, as being a ritual of raising the Djed pillar, performed by the King. The ritual was also incorporated in the Heb Sed festival of Amenhotep III at Waset (Thebes).

There is a depiction of it in the Hall of Wesir, and it is thought that it formed a conclusive part of the Passion Plays, or 'Mystries of Wesir', which were celebrated annually at Abedjou. The ritual symbolized not only the stability of kingship but also the resurrection of Wesir. But this part of the celebrations wefre attended only by the King if he had been present at Abydos, the priesthood and other temple servants. It belonged to the closed part of the Mysteries which were kept out of sight from commoners. So it seems that these Passion Plays were acted out at two planes of existence at the same time. There was the mundane part for everyone, to connect with the mysteries of resurrection, and there was the exclusive part which were for those who were perhap sconsidered more 'tuned in' with the gods.

Sources:
Guide to Religious Ritual at Abydos - A. Rosalie David
The Ancient Egyptians; Religious Beliefs & Practices - A. Rosalie David
The Complete Temples of Ancient Egypt - Richard H. Wilkinson
Cultural Atlas of Ancient Egypt - John Baines & Jaromir Malek