Do you have discounted days ?

None were advertised during July or August. In addition to their memberships they also have AZA reciprocity. So members of any AZA zoo or aquarium can get in any day for at least 50% off, and they give you 10% off at their gift shops. You can see the list here: https://www.aza.org/reciprocity

Some on a monthly basis and ppl buy membership s

They do have some. I get a membership every year so rarely worry about that