It looks like they rarely offer free days. There are no current free days scheduled from what I can tell.
I know they have very rare free days but not sure when or why. If you ever have an injured animal you bring to their hospital they will give you a free pass but idk if they give them for other occassions
You can see quite a bit from the outside at no cost and there is a great outside playground behind the building. One could go see what animals are being shown outside and then go to the playground. It is a lovely area.
Thanks! Your answer is awaiting moderation.