The trend from my school's OCI was also for callbacks to spike Thursday and Friday. A common practice at a firm is for the interviewer to return with notes / impressions plus the original resumes, then a committee to make decisions, then the original interviewer calls/emails the called back candidates. As a result, you have no way of knowing when that committee can get together. Certainly some firms plan to get it done within 24 hours, or authorize the partners going to OCI to make the calls that evening. But at other firms, they might interview on a Thursday and not have the committee meet for a week.
Additionally, it seems to be the case that firms will quickly send snail-mail or email rejections to most interviewees, quickly callback some interviewees, and then do nothing with a much smaller group. That smaller group is in the running for a CB. It isn't very likely, but people get jobs every year from firms which did most callbacks/dings in lat August but got back to this particular person in mid-September or later. Don't get to caught up in the '24 minutes after the interview or you won't ever get a callback' hysteria.