How about both (May and June)? 
(kidding)
The May SAT is May 2nd. AP testing starts the following Monday. So depends on how you and your student feel about potentially studying for both. And the June SAT testing date will coincide with any SAT subject tests that your student may want to take, if any. And there’s finals too.
If your S needs to take it once more, then I’d contemplate June, if no subject tests are needed and right before finals. Wait for more prep time. There’s always August and October too.