(an update of something I posted back in 2012)
September 15 is *not* the ERAS application deadline! Although it is the first day that you can send out your applications, you are not required to do so. Residency program directors will be downloading applications throughout the rest of the month until they can retrieve the MSPEs on October 1, and quite possibly even beyond that date. I know that some programs don't even log into ERAS on the 15th, partially because the system can be incredibly slow on the first day.
Multiple clients and even some non-clients have been asking for advice recently, wondering what to do if their USMLE Step 2 results aren't ready yet. If waiting until September 16th or 17th will make a difference, you can certainly delay your application, and apply with a more complete set of documents all at once. You won't be seen as "late".
Similarly, several people asked what to do about a letter of recommendation that still wasn't available in ERAS. In this case, it might benefit you to send out your application on the 15th without that missing LOR. A late LOR doesn't typically reflect poorly on you; program directors know that even if you request something months in advance, sometimes your attendings just get really busy or distracted. However, you will need to go back into ERAS again to manually assign that LOR to every program that you want to receive it. It's more work for you, but it enables you to get the rest of your application out faster, if it's ready.
Yet there are many other potential applicants who are not ready, and are wondering if they should apply at all. If you're juggling too many things, like final rotations, observerships, research projects, and studying for Step 2CK, then this application may feel like another burden that you're not ready for, particularly if you've never had it professionally reviewed. Many potential applicants should not apply this year, and instead should focus on doing well on their exams and their clinical experiences. People who try to do everything all at once and then don't match get stuck the next year reapplying with lower USMLE scores and weaker LORs from attendings who weren't as impressed as they might have been. Do you really want to spend all of your money on applications this year that have so little chance of success? It's better to develop a strategy for what you should do in the coming months to continue developing your clinical skills, adding relevant experiences to your application, and putting yourself in a position of strength for next year.
There is no general advice that works for all applicants, and a blog like this can't possibly cover all situations. My suggestions for your particular situation might even disagree with what I just wrote above, depending on various factors. Please contact me for advice, preferably sharing copies of documents like your personal statement, LORs, ERAS application, and MSPE, and I'll do my best to give you some personalized guidance.