I also completely agree; this article provides a really good idea to weeding out candidates but weekends are a no go (especially if they don't give you any type of warning ahead of time; at least if there is warning things can be moved around and scheduled). A week is a good amount of time. Since they have to articulate their work when they return it's not like they can cheat; they're going to have to understand it well enough to talk about it and discuss the pros and cons of their design.