Fall Conventions...... City University convention just ended. I think best bet is Rainbow Gala which is in December. I forgot when Hong Kong University has its own convention.
Regarding February to June, there is a Polytechnic University event sometime in late January/early February (advertisements should be around Sino Centre or Ginza Plaza...the places cosers hang out). Also, there's Comic World and another Rainbow Gala between those periods as well. If you stay long enough, sometime in July, there is the University of Science and Technology event.
Honestly, the biggest university event is City University's. I'd say the UST event has better scenery for photo taking while Poly U usually has a lot of cosers.