When do you actually clean?

I have friends whose homes are immaculate no matter what time of day I drop in...either invited or unexpected. What really amazes me is that they never appear to be working...they are always reading or relaxing or talking on the phone. Now I am no Martha Stewart, but I certainly appreciate the work it takes to keep a home spotless. No matter how hard I try, I always am in the middle of folding laundry or cooking or emptying the dishwasher or vacuuming when someone drops by. So my question to these people is, when do you clean?