If you’re only being offered Monday, Wednesday, Thursday and Saturday every week then you need to take either Monday or Saturday every 2nd week off as well, assuming you’re on a higher rate for Saturdays then taking every other Monday will be better for you financially.
If its the same client then maybe a chat with the TM/ person in charge before making that decision as they may help out by looking to book you on a Tuesday, Friday or Sunday instead of one of your current days.
If its for different clients then take the day off from the one you most dislike working for.