What exactly, is a "day"?

I have collected for myself several 'summons' and I'm very interested in trying them out. However, I'm hesitant to do so because they say they can only be used once per day. Well what defines a day? Is this a real-life day or a Southpark day? If it is the latter, then what makes up a Southpark day?

(I suppose I could save, use them and load the previous save, but that would defeat the purpose of asking this question.)

Solution 1:

A day is determined by completing certain amounts of the main story line. Eventually in order to progress you'll need the abilities granted by what you get that following night, so it naturally weaves itself in.