it doesn't really matter what day you work what in isolation, but you have to be aware as a whole. very common is to do three days of weights with three alternating days of cardio-- rest on the seventh day. depending on life style and body groupings i would build your weight days off of that. for people who hate leg work outs, i recommend putting that on day one. you also want to consider what day is best to maximize your rest day. for instance, if you're doing a leg day, you won't be working out arms peripherally; however, you will if you're doing a back day. if you're new to this type of schedule, i'd say put legs in the middle and have a back and arms day and a chest and shoulder day (some combination like that) as bookends so that you get more days in between to recover. also, your fitness activity during the week should help dictate that.