I've never thought of September as Bigfoot Month, but according to Haunt Jaunts' calendar of paranormal/horror festivals, there are three Bigfoot festivals this month, so I guess September is the month to celebrate Bigfoot. First up is the International Bigfoot Conference in Kennewick, Washington, which takes place September 2nd-4th.
That same weekend (on the 3rd), there's also the Bigfoot Celebration in Willow Creek, California.
Minerva, Ohio's Minerva Monster Day takes place on September 23rd and 24th and will focus on Bigfoot, the Loch Ness Monster, the Minerva Monster, and the Beast of Whitehall.
Check out Haunt Jaunts for a list of all of the fun paranormal and horror festivals going on all year. : )