Origins of the Feis
The word Fés [faish], which literally means a feast or celebration, cognate with Latin festum and English feast, was generally applied to the three great meetings of Tara, Croghan, and Emain. These were not meetings for the general mass of the people, but conventions of delegates who represented the kingdoms and sub-kingdoms, i.e. the states in general of all Ireland, who sat and deliberated under the presidency of the supreme monarch.
The Féis of Tara (known as an Aonach ‘great festival’) , according to the old tradition, was founded by Ollam Fodla [Ollav-Fóla], who was king of Ireland seven or eight centuries before the Christian era. It was originally held, or intended to be held, every third year, at Samain, 1st November.
[Source: http://www.libraryireland.com]