Sozin's comet entering the atmosphere, inviting every Firebender to have a burn your nation party.
Sozin's Comet or formerly known as The Comet that can epically PWN the world was a comet made of ice and rock, so I don't even know why the hell that it was able to give firebenders the power of 100 suns so they could pwn the world. But whatever. Also comets don't just fly by for a half-hour. They usually stick around in the sky for weeks at a time.
