It's not too early to start preparing and psyching yourself up for career fairs (or job fairs, career development events--they go under various names). Here's an article that outlines a list of must-haves/must-dos before you attend an event: http://www.glassdoor.com/blog/10-tips-oncampus-career-fairs/
If you want to know what career fairs are upcoming, check here, at the Niagara University Career Services calendar of upcoming events: http://www.niagara.edu/career-services-calendar-of-events/. Extra bonus points: many of them are on campus, so you don't need to hop a bus or beg a ride!
When should you go to such an event? Well--yes, obviously, when you are actively on the market, ready to graduate, and looking for a job. But here's an insider tip: go anyway if you are still years from being ready to roll. Shake hands, walk around, get a feel for how the whole thing operates. That way, you'll avoid that glazed-eye "deer in the headlights" look when the real thing is an inevitable part of your new life.