It's really a good concept, setting aside a special day for spiritual renewal. The community or fellowship understands the importance of the day and give it particular attention in order to meet with God.
So what about Sunday? At the Training College, a week day is chosen so as to transform a "regular" day to being marked by this holy ambition. It has merit. Is there an application for the rest of us? There could be. Maybe you might take a personal day from work to have a spiritual day where you totally focus on the Lord, purposely limiting any and all distractions. I think it would be worth it. Why not do it with a couple of friends?
2 Peter 1:5 (MSG)
5 So don't lose a minute in building on what you've been given, complementing your basic faith with good character, spiritual understanding,