If it were to do about the HSC it would be boring. I went to a lecture last year, and we've already covered everything in it in class. The reason to go to the day is because the first two lectures may be interesting, assuming you choose a topic you know nothing about. The AM-GM lecture might help for HSC, although most people who know it don't use it, they're too scared the stupid markers won't know what the heck you're doing. But its a day off school, why not go?