Here's a simple list of courses.  Try asking it for a course that's on Monday or a day that phil250 tales place.

# Try: [Class monday ?]
# [Class ?day ?course]
# True (succeeds) when ?course takes place on ?day
[predicate]
Class monday math300.
Class monday phil250.
Class tuesday cs396.
Class tuesday rtvf376.
Class wednesday math300.
Class wednesday phil250.
Class thursday cs396.
Class friday math300.

Now try writing a query that will tell you a day of the week when there is no class.  Note that you can check if there is a class on day ?day by saying [Class ?day ?] and if there is not a class that day by saying [NotAny [Class ?day ?]].

# [Class ?day ?course]
# True (succeeds) when ?course takes place on ?day
[predicate]
Class monday math300.
Class monday phil250.
Class tuesday cs396.
Class tuesday rtvf376.
Class wednesday math300.
Class wednesday phil250.
Class thursday cs396.
Class friday math300.

# [DayOfWeek ?day]
# True (succeeds) when ?day is a day of the week
[predicate]
DayOfWeek monday.
DayOfWeek tuesday.
DayOfWeek wednesday.
DayOfWeek thursday.
DayOfWeek friday.
DayOfWeek saturday.
DayOfWeek sunday.