How can season 3 take place during the 초 semester after season 2 when both have a 할로윈 and 크리스마스 episode?

This is something that constantly puzzled me. For a while, I've gone through a theory that due to a spell gone horribly wrong, Sabrina and everyone else in her area were forced to go through the same 년 twice while still having knowledge of the 년 before the incident.