As an HR manager by trait...second interviews are not always established the same day. It most likely will be with another person and their schedule needs to be checked, etc. Now, the fact that they called you is def a plus on your side. As others have said, def send follow up email thanking for time (it does count) and also the fact that you were told to call was a good thing too. Most importantly, keep in mind that this is a holiday weekend and things are prob moving slower than usual for the company. Best wishes