Full-Time offers

So I missed my opportunity to attend a big 4 (or top 10 in general) summer leadership conference last summer to have the opportunity to get an internship for next summer. Now I'm going into my last year of school hoping to be able to get a full time offer from a big 4 firm. Do any of the top 10 firms invite full-time candidates to their summer leadership conferences? Or do I have to wait until fall to compete for one of the few leftover positions from the summer/spring interns who turned down their offers?

Thanks.