History

Soon after Aang disappeared 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 extinction of the Air Nomads, the sole exception being Aang. The comet was renamed "Sozin's Comet," in honor of the Fire Lord's great victory.
In the episode "Winter Solstice Part 2: Avatar Roku", Avatar Roku warned Aang of the comet's return in the next summer. With its passing, the Fire Nation would easily finish their world conquest. This gave Aang less than a year to learn Waterbending, Earthbending, and Firebending, as the Fire Lord's power at the comet's passing could presumably match a fully realized Avatar, much less the partial knowledge and experience Aang possessed at the time. 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.