I applied a little late last year. I didn't recieve near as many interviews as expected and now with no acceptances I am preparing for round 2. I am curious as to what month each successfull applicant returned most of his/her secondaries.
I have heard about success stories from people who applied as early as possible or who waited until the night a secondary was due to submit it. The conventional wisdom is the earlier the better (as you no doubt have heard)
Personally, I returned all of my secondaries within 48 hours of receiving them, and this was after submitting my primary on the earliest possible day and having it verified in early June. Most secondaries come out in July, but there are some in late June or August. I got 10 interviews, 4 acceptances (and still waiting on 2 waitlists, withdrew from 2 waitlists), which is more successful than I really imagined that I would be.
Good luck! Complete things as early as possible (without rushing)!