Maesters wouldn't be needed if it the seasonal changes were determined that easily. Do you have a citation from the book that they used day length? That's not consistent with the variable star theory, and if you're right then it's likely that the author didn't have a valid scientific rationale for it.

This article doesn't really answer the question about season length. To me, having a background in astronomy, the answer is obvious. The only likely rational possibility is that the planet has a small obliquity of the ecliptic (axial tilt) that does not cause seasonal change, and revolves around a long-term irregular…