Check the zoo website for any special events, like a trainer talk, a feeding schedule, hours for special exhibits, and more. Most schedules are standard through the day or week, so they'll be posted and you can plan around those. Getting to the zoo earlier in the day means its a little cooler and there's a better chance that the animals will be more alert and active. In the middle or end of the day, when it's really hot out, the animals tend to get tired and don't interact as much compared to when they feel comfortable (weather wise). If you get to the zoo when it opens, and allow yourself to stay the whole day, you can go at your own pace and really enjoy the experience. If you show up later in the day, you might feel rushed and not enjoy it as much.