Panathenaea was an important annual festival of Pallas Athene that according to tradition dated from the time of the legendary king Theseus. From 566 BC Pisistratus, the Athenian aristocrat, arranged that every fourth year, when the festival would be known as the Great Panathenaea, music and poetry competitions were included, together with games and the festival was extended by three or four days.
During Panathenaea there was a solemn procession to the Acropolis in thanksgiving to Athene for having saved the city, giving it victory over the ‘nation of Poseidon’. This has been seen as a clear reference to the destruction of the Atlanteans. The fact that this ceremony was performed at least 138 years before Plato was even born would appear to demonstrate that he could not have invented the existence of Atlantis. If ‘the nation of Poseidon’ is not a reference to Atlantis, what does it refer to? One suggestion, although equally contentious, could be the Sea Peoples.
The contest of Athena and Poseidon in the West pediment is related to the later conflict between Erechtheus, an early king of Athens, and the Eleusinians under the leadership of Eumolpos. Keep in mind that Poseidon is the father of Eumolpos, dated by some to reigning around 1400 BC.
{19.04.11}New research by Professor Efrosyni Boutsikas of the University of Kent has shown(a) that the start of the annual festival was signalled by the appearance of the Draco constellation over the Erechtheion on the Acropolis of Athens.
(a) http://www.unreportedheritagenews.com/2011/04/rising-above-acropolis-constellation.html

