On the first day of the festival, a great procession went from Karnak to Luxor. Amun-Re, carried on a veiled bark by waeb priests, made a visit to the 'inner chambers' (Opet of the South, Ipet-resyt) to Amun at Luxor.

The Barque of Amun, relief from Karnak.

In Hatshepsut´s reign, the god´s statue was carried in his bargue by waeb priests by land to the Opet temple. It travelled back to Karnak on the river, on the god´s ceremonial barque, which was escorted by the royal barque and the king himself. Later, after the Amarna period, the festivities covered about 27 days. The whole Theban triad of Amun, Mut and Khonsu was towed southward on their barque, both by boats under sail and by men at ropes along the shoreline. The procession was followed by dignitaries, dancers, singers, musicians and common people. Priests carried incence, sistra were shaken, there were singing and clapping of hands. All along the way there were probably merchants and peddlers offering wares to the people gathered on the shores to catch sight of the bages. It was the greatest Theban festival during they whole year.

The King first offered at Karnak to Amon-Re and to Mut, and promises that he will renew the god´s cult. Next the King offers incence and water before the festival boats of Amon, Mut and Khonsu, which were still resting in Karnak.

From there they were carried, in the company of the King, to the quai, where they are put on big barges, and towed and under sail, they travel to the Luxor temple (Ipet Resyt).

At Luxor, the god was greeted by high dignitaries, sacrifices were made, and there were several offering booths along the way from the river to the temple. There were also acrobats and musicians performing for the god, as well as temple personnel out to welcome him.

When the procession reached a triple shrine, it halted. Here Amun, Mut and Khonsu were brought to their respective shrines inside the temple. We don´t know all the content or the meaning of the rituals which were now performed inside the temple. It might have celebrated a sacred marriage in order to certify the ruling king as a true son of the gods. Further it constituted the renewal of the King´s right and power to rule, as he entered the temple. The coronation rites were repeated at this festival. The King received the various crowns and through presenting special offerings, he was renewed by the god. In this way, the Opet Festival served both as a renewal of the god and the king, plus it emphasized the bonds between the gods and the people of the land.

Sources:
The Complete Temples of Ancient Egypt - Richard H. Wilkinson
Article in AEgypten, die Welt der Faraonen - Regine Schulz, Hourg Sourozian