When ACT is curved: after each session or once per year? Probably, after each session. Then, what session is the best? Winter, probably, when all procrastinators take the test. Autumn is, probably, the worst, as the strongest students were studying all summer for this test.
There is no “best” season according to your logic. The test is designed so that it doesn’t matter when you take the test. The scales are NOT affected by who is taking the test on a given test date. The scales are predetermined based on a previous entire year’s sample size.
You can have an easy test in any given date. Or a hard test. But the scales are set up so that your score should not be too much affected by the difficulty of the test on a given test date.
I would recommend June. Your brain is still fresh off of the school year (school is over where I live at the end of May), and you have a couple weeks to study hardcore before the test. I think ~50 kids took June at our school and 2 of us made 36s. I’m a little biased, but I think it’s the way to go!