Do ACT tests vary on different test dates or are they the same?

Are the ACT tests given on different dates in the same testing year the same or do they vary? For example, are the Sept. 2015 and June 2016 tests the exact same test?

No, the ACT tests given in the same year are not the same test (so September 2015 would not be the exact same test as June 2016). While there is some anecdotal evidence that ACT, Inc. may occasionally reuse international tests in the US (and vice versa), it is unlikely that ACT, Inc. would reuse any test that is available through Test Information Release (which includes any tests administered in June, December, and April).