Even today there are "history" professors that teach the history they like and "leave out" what they don't

Exactly! More than the professors, though, it's usually some mandate from above that doesn't allow the teachers to teach what they want. This calendar thing itself is kind of meaningless. I mean it is just trivia and isn't so important to know. But it does hint at the larger issue which is that history isn't considered very important and isn't taught well, nor related topics like civics.