Is "hour of the day" where the value can be 0, 1, 2, ..., 23 a categorical variable? I would be tempted to say no, since 5, for example, is 'closer' to 4 or 6 than it is to 3 or 7.
On the other hand, there is the discontinuity between 23 and 0.
So is it generally considered categorical or not? Note that 'hour' is one of the independent variables, not the variable I'm trying to predict.