I know the one you mean, @Russ-Van-Dine I saw that too and was looking forward to it, but it seems to have disappeared. There's an explanation of the challenge in Will's Creative Composition class, he says he sets the challenge for his college students every year, you have to fit 50 items into a single composition.
You can do the challenge yourself, by working through the Creative Composition class and then having a go at that challenge - I think it's near the end of the class.
When I saw it on the calendar I thought maybe a group would be doing it together and sharing feedback etc which would have been cool. Doesn't look like it's going to happen now though.