That's a good question, Terry.
January is usually kinda lame. HOWEVER...it's also the month where the "limited released" Oscar pictures get a wide release. And its also the month that people check out the movies that were already in wide release but ignored till the Oscar nomination.
February is a bad month for guys, but girls like it cause of all the romantic comedies being released cause of Valentines.
March used to suck, but 300 changed that.
April used to suck, but QT and Robert Rodriguez changed that.
August is really not THAT bad. Yea, all the "big" movies have already been released by this point but you're still gonna get stuff like Superbad and the Bourne movies.
September DOES suck. I can't think of anything good except some indie flicks here and there.
October is horror month. All the horror movies are released during that month. So if you're a fan of the genre then you'll enjoy that month.
So...I'll go with September.