Here is my answer in white text to avoid spoilers:

A Day-Knight would always claim that it is day, saying so truthfully during the day and as a lie at night. Vice-versa for the Night-Knight. Our Knight has revealed himself to be the latter by announcing that it is night, so he must be lying when he says he is a Day-Knight. Since he is lying, it must daytime so I would not cast my spell.

The real question is, which goblet contains the iocaine powder?