Karnak's largest festival emptied the temple rather than filling it: once a year the god himself left the building.
The barge leaves the sanctuary
During the Opet Festival, held at the Nile flood, priests carried a portable barque shrine holding the cult image of Amun out of the innermost sanctuary at Karnak, joined it with the images of his consort Mut and son Khonsu, and processed the group down the Avenue of Sphinxes to Luxor Temple, later also by river. The pharaoh went too. Inside Luxor Temple, ritual renewed the king's divine authority for the coming year, a re-coronation performed out of public view inside a temple built for exactly that purpose.
The road between the temples
The avenue connecting the two temples, lined with sphinxes and rebuilt or extended under several dynasties, fell out of use and was built over for centuries. Egypt's antiquities ministry cleared and restored the full processional route, reopening it in 2021 with a nationally televised event, so the road the god's barge once travelled is walkable again.
Why it mattered
Opet was not spectacle for its own sake. Kingship in this theology needed periodic renewal through contact with Amun, and the festival was the mechanism, staged along a route built to carry it.