The cheapest day of year to buy plane tickets is actually today, August 24!
With most people finishing summer vacation and school starting back up, autumn tends to be a fairly dead time of year for trips so airlines try to lure flyers by introducing sales in late August!
In fact, if you're planning on traveling before the end of the year September through November is the best time of year to save some cash! You get non-scorching weather in most of the Northern Hemisphere, and smaller crowds than you would in spring and summer.