Halloween Horror Nights at Universal Studios Florida started in late September, and it runs through November 2 this year. The event is open on Halloween night, and common sense would seem to dictate that the holiday would be insanely busy. However, that's not always true.
It's impossible to predict the exact crowd level, especially without knowing what the weather will be like since rain drives people away, but here are some considerations this year:
1) Day of the week. Halloween falls on a Thursday this year, which means a lot of people will be at school and work. If the event gets crowded, that likely won't happen until later in the evening. If you can get there early, you'll enjoy short lines until everyone manages to get to the park.
2) The holiday. People with children are often stuck taking them trick or treating on Halloween. That means getting to the park late or not at all. However, that doesn't impact older school age kids and childless adults.
3) The intellectual properties. This year, Halloween Horror Nights has been busy in general, and much of that is due to popular franchises like The Walking Dead. The Walkers were there last year, too, which contributed to a much heavier than usual holiday crowd.
5) The tickets. Thursday has the cheapest ticket price, so that might entice more people to come on the holiday. Also, locals with Frequent Fear tickets can get in on Thursdays, so they're likely to take advantage of their last entry day.
Those are the five main factors to consider in determining just how crowded the park will be. Hopefully they'll help you decide whether a holiday visit is worth your while. If you come on any night, I recommend reading and following my touring plan in this article.