Basically one or more members decide they want to host a theme week or weekend and set dates amongst themselves. Then they use some of their submissions to promote it ahead of time. It's hard to pull off though unless you're a top memer and or you make friends with top memers and get them to help promote it. I waited until after I was on the leaderboard for a while before hosting a theme week, and made sure I got Raydog, Isayisay, and a couple others to help promote it. But it was a big success. But anyone can do a theme week if they want