Toby Jones, PlanetSide 2's communitz manager said that the biggest battle took place on September 6 with a total of 576 players joining. Of these, 371 were active simultaneously. All these players met on the battlefield prepared directly for the event and 2 teams had 2 hours to earn the most points by capturing territory. During these two hours 27.000 deaths were registered.
However such events are not needed if you want to be in the middle of similar fights. They take place everyday and you can just jump to them any time.
More interesting information is written in the article linked below.