Young brother, your questions are wise and telling. For both sake of tradition and practicality, the spellcaster's battle will be held much like in previous years. All will wage battle simultaneously and the battle will commence shortly before evening repast, shortly after the hour that most of the learned study their lore. This provides the most opportunity for each participant to be at their prime and the least chance to interfere with the battles that may occur during the average day of heroic adventurer.
As a reminder, high magic and the spells of the highest circle are permissible this year, but rituals and magic items are not. Also, this is a competition of individual heroes, not groups, so none should collaborate before or during the competition. From the tales that the trees and grass have spoken to me, I have no doubt that the honorable heroes of Ashbury will maintain these standards.
I hope that helps you prepare for this competition and hope you will participate in other competitions as well. Should the future bring changes to anything I have said, your dreams will fill again with more of my prattling.
Master Kerios