Dear aProgDirector:
Thank you for your valuable input. The reason why I can't include a LOR up front is because I will not have a letter from an EM director until the end of august or september (My school do not allow us to do an EM rotation until we're in our 4th year). So, given that situation, let's say that I have an EM elective in August and one in October. And with experience, I perform really well in the October rotation and got an excellent letter from an EM director at the end of my rotation. By then, my application might have already been ruled out by many programs due to lack of strong endorsement and will not be re-reviewed.
So the dilemma is, should I turn in my application as soon as possible, even though I know for a fact that my application can be improved with stronger endorsement in the later months? Or should I hold and apply late so I can wait for the LOR?