Tau looks like r, which is used for radius length, which is relevant to some equations that use the circle constant. Of all possible letters, why use tau? Are they trying to mess with students and teachers whose handwriting makes it ambiguous whether a symbol is tau or r? Maybe it was just because it's half the pi symbol.

I'm pretty sure people only care about Pi Day because it sounds like pie. Tau Day will never be as celebrated, unless you call it 2Pi Day, which defeats the point, but is still a good excuse to eat pie.