This may seem like a silly question, but I want to make sure that I do not back myself into a hole.
@CameronBameron is exactly right! You can definitely apply RD before the ED deadline, but the main reason many don't do that is because they do not want to pay the RD application fees until they receive a decision from their ED school.
Yes, you can. If you are accepted Early Decision, you will then have to withdraw your application from your RD school (s). One of the reasons most applicants do not do this is because they don't want to waste $75-90 per RD application, if they get into their ED school. So if you applied to 10 RD schools before 12/15 when ED decisions come out, and you got in ED, then your parents would be out $750-$900 without refunds.
If you are first-gen/low income and have fee waivers for all your schools, then this is less of a concern but you will still have to withdraw your application if you get accepted ED.
You could do this, but my question is: why would you?
Many colleges say that applying long before deadlines gives you no advantage in the application process. Although it can be nice to get off your plate and out of the way, those extra days (or in this case, weeks to months) of having the ability to edit or revise your application can go a long way. Even one extracurricular added or typo found could make a big difference to admissions officers. In this extra time, you may even be able to get an extra recommendation from an employer to help bolster your chances, or join a competition and win an award. In my opinion, it's easier to just submit everything together closer to the deadline.
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