In your case, it's fine to push back your Step 2 CK to a later date. In general programs do not have a hard requirement for Step 2 CK for offering interviews (a couple might, but that would be the very rare exception). I usually advise students who did well on Step 1 to take it when it is convenient for them. For students who did poorly on Step 1 and reasonably think they can improve their Step 2 CK then it is definitely worthwhile to take it early. September, October, or November... all fine for you to take your Step 2 CK.