When precisely is the Beginning of the game? while this could be seen as a philosophical question, the newest set actually makes it a bit important.

while it almost never matters, the timing is Important to know exactly when interacting with the phoenix force resource.

i believe that the two best guess options are:

1) just after the Roll off to see who goes first (to establish priority) and thus just after revealing your forces.

2) at the beginning of the first turn of Play.

option 2 does seem to be the more likely choice, but we have traits that begin working before the game begins (typically things like Duhg's army) and of course effects that trigger when placing your forces, which if 2 is the BotG is also now before the game begins.

option 1 goes with the theory that the game is truely begun after the players have revealed their forces, and most importantly, done that first and sometimes critical action of the game: the roll to see who goes first, and who picks the map.

It is after all figures have been placed and before the first turn.