There's a lot of confusion about application dates, so I want to say this as clearly as possible:
September 1 is not an application deadline, so you do not need to submit anything through ERAS that day.
It's true that you could certify and submit your application on September 1, but there's no benefit, and potentially you might limit yourself. For example, imagine if you submit your application that day, but then you want to add something new before the programs can read it? Too bad; there's no way to do that. For example, if in the first week of October you suddenly participate in a one-day health fair, but you already certified and submitted your ERAS application, there would be no way to add that as a new Volunteer Experience.
October 21 is the actual first day that program directors can download your ERAS application this year, and all applicants should be completely finished well before that date arrives. Here is some helpful information about the schedule. You can see there that the residency programs can't start reviewing applications until October 21, even if they were submitted much sooner. (In case you're thinking that the date "September 15" is important, in the years before the pandemic, that's when programs could review your application; this year for the first time it was changed to October 21)
To use an analogy, imagine that you're standing in line at the U.S. Post Office, with your application in a sealed and stamped envelope. Applying on the very first day would put a September 1 postmark on your application, and that's when your application would get mailed to the program directors. But then the envelope would sit in a pile for a month and a half, gathering dust. The program directors can't open the envelope until October 21, and all the envelopes will get opened on the same day, whether they were mailed on September 1, October 21, or any day in between. Also, according to the schedule link above, "Applications submitted on or before Oct. 21 will display an application date of “October 21” to programs" -- so the programs won't even know that you tried to submit your application early.
As for the NRMP, that's almost completely unrelated. ERAS transmits your documents to program directors (who can't read them until October 21), but the NRMP coordinates the Match in February / March 2021. Although you should register early with the NRMP to get the best price, the Match is not something to worry about right now. By the way, the Personal Information section of ERAS is the only part of the application that can be changed after you certify it, which means you can update details like your address, ACLS certification, and your NRMP identification number. But your Experiences, Publications, Education and almost everything else can't be changed after you certify and submit your application.
In summary, don't rush to finish everything this week! Take the time to finish your application the right way. As always, please contact me if you have any questions about the residency application process. I am happy to provide a free assessment of your application. I have twenty years of experience in helping people develop their personal statements and ERAS applications, prepare for interviews, learn about what residency directors will think of their strengths and weaknesses, and much more!