I think anytime is a good time. Fall and spring will have better temperatures, but can also be more rainy. In the summer until Labor Day the zoo is open until 6 p.m. There are plenty of indoor and outdoor exhibits. The animals are usually most active early in the morning and late afternoon.
The animals are most active on days when it is comfortable outside temperature wise so late Spring or Fall would be my suggestion. October is always fun because they have the HallZOOween event where the animals play with pumpkins and the kids get to dress up with lots of other fall activities all over the Zoo.
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