After a week of working to immediately go out, either for some specific reason or just chasing it, is really unenjoyable. Everyone's got this horrible post-work aggro vibe. Much better to get a decent cook on, do some exercise, watch some films, listen to some music, get some sleep. Thus leaving you completely refreshed for Saturday and all the fun it contains, both during the day and in the evening. A big Friday night ruins your weekend.
Saturday is the best night to go out.