if you want to take the September 10, 2016 ACT, you have to register by Friday, August 5. You can register with a late fee from August 6 to August 19. You can check the details here. You should also be doing ACTs under timed conditions, including the essays - see my other blog entries for hints on the SAT and ACT.
By the way, the registration deadline for the October 1, 2016 SAT is September 1, and the absolute late deadline (phone or online) is September 20. The mail-in date is September 13. You can check it yourself here.
Author: John Linneball Who did you think? ;-)
I'm the proprietor and only tutor for this business; that's why I named it after me.