Gamma prety much nailed it, with a couple minor corrections. ERAS opened August 15th, and Sept 1st was the first date we could submit the application to the programs we applied to. In other words, you have 2 weeks to complete the application before the earliest submission date. I didn't get the impression that it was necessary to have your app in on that date, but I also regret not having my app in on time just in case it does matter to a few places. Certainly, as I mentioned previously you don't have to have all your letters in at the same time you submit your app, but again, why take the chance of making more work for the coordinators and pissing them off if you can help it. By the way, it was tight getting my Sept rotation LOR in, while I know people have done it, I wouldn't have wanted to stress about getting an Oct. LOR in on time (some places like Case Western and UIC, start sending out invites in mid Oct.).
Regarding who to get LOR's from, there is some disagreement. Everyone agrees most of your LOR's should be from ortho attendings, but a few places require some other letter (med attending, gsurg attending, resident, etc.). Others people say that one of your LOR should be from something outside of ortho, even if they don't specifically ask for one, to show you're more well rounded. Med attendings are notorious for writing excellent letters, so I went that route.
Good luck. This whole process is a pain, but as long as you're prepared for that, it's tolerable, perhaps it'll even be a little fun.