The August tradition in Europe might go back to when humans first started planting there. In ancient Greece there was about a six-week window around August/September where people didn’t have to constantly work the farms. That’s when they’d suit up, go into an enemy’s territory, and start hacking at the grapes to piss the other guys off and get them to come out to fight.
They might maneuver around for a few days but they usually found flat ground on which to fight, because either side could just walk away from the fight if it wasn’t even. There wasn’t much time because it took so long to get ready and do all the marching, and they had to plan to be home in time for the harvest.
They’d have an astonishingly violent battle that always ended within the day, and then the survivors would go back to work.