Ile-Ife holds the festival because Ile-Ife claims Orunmila. In Ife tradition the figure called Agbonniregun, identified with Orunmila, lived and taught in the city, and the shrine at Oke Itase is where that claim is kept. The Ooni of Ife attends, makes his offering and sits while the casting is done, which places the palace inside the festival without putting it at the centre. On Olojo the arrangement is the other way round.
The June date is worth stating carefully, because a claim rides on it that is easy to overstate. Current practice sets the casting on the first Saturday in June and treats the announcement as the opening of a new Ifa year. A good deal of contemporary Yoruba revivalist writing goes further and presents that day as the Yoruba new year in a reckoning called Kojoda, carrying a year number in the ten thousands. The festival and the June casting are well attested. The year count is a twentieth century reconstruction, is not attested in the older record, and this atlas keeps the two apart rather than folding one into the other.
What is old is the shape of the thing: a divination performed for a whole community at a turning point, by the senior diviner, with the result published as guidance and as a list of sacrifices. That pattern is documented across Yoruba towns, and the annual casting at Oke Itase is an instance of it, whatever is counted from it.