I'd say Augst 17 or April 7.
Just kidding of course :)
So, if you are recruiting out of cycle (i.e. not with a school), you can genuinely apply at any time. Now, when you apply depends on when you feel you have the best odds. This ties closely with networking.
So, start networking now. Then, as you get referrals, or get put in touch with HR, apply to those firms! You don't need to apply to every firm all within the same week!