It is always busy.
It is much busier in Spring Break which is the last two weeks of March 2018 until 4/2/2018. It is also much busier during School Summer vacation. On school days there may be the elementary graders (and kindergarteners with patrons) group visiting the zoo; But overall if you come at in the morning (free parking for members already open half an hour in advance of opening time); Say you come at 9.30 a.m , you are all set for the best time having fun at the zoo. Most groups (as far as I experienced myself) do not yet come before ten in the morning. Enjoy your visit to the zoo.
Yes lots of kids
Busy on school days until about 2pm then it is fine.
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