I think it's by August the scholarships need to be at 85 total.
August 1 is a good approximation. You can't have more than 85 counters for the season. This is how the season's start is defined:
17.11.2 Length of Playing Season. [FBS/FCS] The length of an institution's playing season in football shall be limited to the period of time between the start of preseason practice (see Bylaw 17.11.3) and the end of the regular playing season (see Bylaw 17.11.5), except as provided in Bylaw 17.11.7 (see Bylaw 17.11.4 for restrictions on first contest dates).
17.11.3 Practice. [FBS/FCS] 17.11.3.1 First Practice Date. An institution shall not commence official preseason football practice sessions, for the varsity, junior varsity or freshman team, before 31 days before its first scheduled intercollegiate game.
So I think you have to be at 85 no later than 31 days before the first scheduled day - i.e., the first day of practice.