S19? Is he a HS senior or junior? For us the timing is all about scheduling around other busy times (finals, busy EC times, etc.) My D21 took tests in June and was not happy trying to prepare for tests and also finals (results were not great). She also took the October exam (no result yet obviously) and had a lot more time to prepare, but had not had a math course for several months so she felt stale on that subject. She’s thinking May for her second attempt. Will be several months into her next math course and timed between trimester finals. If she was a AP student May would be a terrible choice as the AP exams are all in May. March to May seems tight to me-- not much time after getting results to actually try and improve the score through test prep activities.