I have used these forums for some very helpful advice regarding ERAS applications, so I want to share my experiences. FYI, I am a senior AMG currently interviewing for Med/Peds in predominantly the midwest.
Yes, Sept 1 is the day the NRMP starts accepting applications, but it is uncommon that applicants have all of their application materials together by then. Some programs will not offer interviews until there is a minimum amount of application materials that have been received by ERAS. All of my interview invites came in late September/early October, and this was with 2 LORS, CAF, and step1/2 scores available to the programs. So it would behoove you to have your application complete ASAP regardless of specialty.
You can also get on the NRMP website that has a publication called the Program Director Survey. This gives specialty-specific information about criteria that PDs use for offering interviews/selecting candidates. It also reveals the percentage of interviews offered before/after the Nov 1 MSPE release.
Step 2 scores are almost always reported 3 weeks after your test date, so you could conceivably take it in August and have them available for ERAS on Sept. 1.
For LORs, one of my letter writers took 2 months and thus I missed the deadline for one of my programs, so no interview. If I could do it over again, I would have started getting letters in May/June instead of August. Also, make sure you look up deadlines for each program you intend on applying to as these will vary.
In the end, your circumstances will determine your own timeline, so just relax and enjoy the ride. You will end up where you were meant to be.
Good luck.