There is a cutoff for Step 2 where you have the option of reporting it or not after you submit the application. I think it was like mid-August. Before that it gets reported on the score report when programs get your USMLE transcript. I assume you want to take it to boost a lackluster Step I performance?
With regard to apps they open in September. My year it was Sept. 1. Technically, you can continue to submit up until January, but you should really have everything submitted within the first few days it is open. Programs start offering interviews immediately upon receiving applications. There is a finite number of interview slots, so every one that gets taken before you submit your application decreases your chances.