Yeah, what Troy said is right on. Mondays and Saturdays are the bigger nights for 40k, although games are played every night.
How common are games on Saturday?
Well it depends on the Saturday really... and whos around. Games are played every Saturday without fail. Generally people just come down, hang out, and a game or games will break out. They can happen at any time really. Sometimes we see 4 of the 6 tables with games on them and the other two tables full of peoples stuff. Other times theres only 1 game going on at a time.
Your best bet, by far, is to post on the forum with dates and / or times, your army, and the point total you would like to play your game at. I don't imagine it'll be tough to find a game.
-Chase