"Hamevazzeh es hammoadim, k'illu oveid avodah zoro."
This admonition re: scorning the moadim seems rather harsh, extreme, maybe a bit over the top!
As a youngster I attended the Shabbos Afternoon Talmud class given at shul by our local rabbi. I was there for most of Makkos. When he taught this, he put it into historical perspective given the times of the Talmud.
Apparently when the early Xtians first broke away from Judaism one of the first "victims" to get jettisoned from their sect were the Holidays. Shabbos was preserved - albeit on a different day, but the Torah ordained festivals were dropped.
Thus, one whose demeanor was to treat the Moadim as "just another day" was certainly suspect as being a "min" (heretic) and was at least guilty of mar'is ho'ayin if not something more substantial.
This makes the apparently "over-the-top" equation quite realistic for them