Hi Greg,
That depends. Come next year, Universal's newest attraction will be Despicable Me: Minion Mayhem which should open in Spring 2014. Should you plan a vacation in May, you might be visiting Universal in the midst of the Spring season and with Despicable opening, crowds could get a bit high.
That being said, the Universal does close relatively early in October to make way for Halloween Horror Nights and the entire park is dressed in a ghoulish theme. If you're bringing a family along and you're not interested in the Halloween event, May could be the best time to go. The park really isn't family friendly during this period.
Either way, if you arrive early and plan accordingly, you shouldn't have a problem doing everything the park has to offer in one day.
Hope this helps!