What ever day you go, be there at opening!
The earlier the better if a weekend. Otherwise any weekday is good.
We went on a Tuesday. It was very nice as there didn't seem like many people were there.
We went on a Saturday morning to be there when they opened. The first hours there were pretty light crowds. The traffic picked up as the day went on, but the place is huge so it never felt overcrowded. The first animal shows started around 10:30-11, everything else is available as soon as they open. I imagine Saturday is as busy as it could get. So honestly, it doesn't seem like there's a bad day to go. You just want to have plenty of time to see everything.
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