I believe it would be most difficult to keep the old Biblical Sabbath. There were many rules that would be hard for today...no driving, no cooking, etc. Now that would be like a retreat but I am not so sure that modern day keepers of the sabbath keep it all. It seems that they drive to chruch and such. So, maybe if we choose to keep the whole sabbath we would be in retreat each Sabbath. I do love the closeness to God at a retreat. How many keep the whole sabbath idea without making it a salvation issue?