Sozin's Comet

History
Sozin's Comet is a comet that seems to pass close the world of the Avatar every hundred or so years. The comet passes close enough that Firebenders are able to draw power from it as they normally do the sun, so their power is approximately doubled.

Soon after Aang disappears a hundred years prior to the series, the comet flew near the planet. Fire Lord Sozin used the comet to drastically raise his nation's power and deliver a crippling first strike in the Fire Nation's war of world conquest. This first strike resulted in the presumed extinction of the Air Nomads.

In the episode "Winter Solstice Part 2: Avatar Roku", Avatar Roku warns Aang of the comet's return in the next summer. With its passing, the Fire Nation would easily finish their world conquest. This gives Aang less than a year to learn Waterbending, Earthbending, and Firebending, as the Fire Lord's power at the comet's passing could match a fully realized Avatar, much less the partial knowledge and experience Aang possesses. There is also the fact that with the comet, the Fire Nation could finish the war, and once the war was over, the Avatar would have no chance in restoring balance.