As early as possible. Theme parks are huge with tons to see and do. If you jump on it early, you’ll get to see more. But there’s no rule about this though. If you’re more casual, then go later. It’s all a personal preference. If you are spending several days at an amusement park then some days you might prefer to start early and end early. On other days you might want to only go in the evening for the nighttime shows.