Technically, Saturday is my official rest day. However, since the start of my program, Friday has been my day of rest (in the most literal sense). Originally I planned that I would do all my workouts in the morning before work. In reality, I end up doing KenpoX Friday afternoons. This makes it possible for me to obtain large amounts of extra sleep at little cost to my daily routine.
Prior to P90X I would go to bed at 10:30PM and wake up at 6:45AM. Lending me a whopping 8.25hrs of sleep. With P90X I've been going to bed around 9:30PM and waking up at 5:15AM, cutting my sleep back by about 30mins (0.5hrs). So when Friday comes, I like to go to bed nice and early (about 9:00PM) and wake up at my regularly scheduled time (6:45AM). This recaptures my lost sleep from the earlier part of the week and leaves me feeling absolutely fantastic!