The early bird gets the worm...and the jobs too! When attending a job fair, the sooner you come the better. It never ceases to amaze me when job seekers stroll into a job fair at 2 pm and the job fair is over at 2 pm - or coming at 1 pm is still too late.
By then, the recruiters are ready to go home! You want to get them when they are fresh and ready to go. After two or three hours of talking to job seekers, they get worn out and are ready to call it a day. So if you come along later, the recruiter is not at his/her best. The only exception to this rule is if there were 2 or 3 recruiters working a booth and they have time to rest and take a break from time to time.
So come early, come often and get hired!