Summer is a popular time for ACT prep because students aren’t in school, so they can focus on the exam. But it can be difficult (and more expensive) to find a good tutor in the summer – the best ones tend to get booked up early.
If you’re self-prepping, I’d say summer is probably your best bet – it’s hard to argue with the ability to focus solely (or at least mostly) on prep. If, however, you plan to get help from a tutor, I recommend taking the April ACT (junior year, of course) and following it up with the July ACT. The July administration starts summer of ’18. You should continue prep in between, and your tutor for the April test will likely stay with you through the July test (assuming he or she did a good job – otherwise, find a new one!)