My first thought is to call them tomorrow. Mainly because they shouldn't be leaving you hanging over the weekend. It is kind of common to get back to someone before the end of the week, but sometimes they might be procrastinating so a little heads up to let them know you are waiting would be fine. Usually about this time my negative defensive thoughts start kicking in. "Who needs your friggin' job anyway?" That is typically right before the call comes in to offer me the job and then it's all sunshine and roses from there. 😀