It's a day when teachers (are supposed to) go on courses and seminars, so the kids get the day off school. Might just be a Canadian thing, I dunno.
Canada is great. When I and some friends first started playing D&D around grade 7 (25 years ago, geez I feel old), one of our teachers set aside an afternoon for us to teach the whole class. We had about five games running, as I recall - I remember being really nervous because the teacher herself was in the group I was DMing. A few years later in high school (grade 11), one of my English teachers asked to join our D&D group; he'd read about the game but never found anybody who played until he heard us talking about it. Played a psychopathic Dwarf fighter, IIRC. I might just be lucky, but I was never treated badly by teachers and such for D&D.