Your application to programs will start to go in September 1. That means, you'll need letter of Recs before then and your app will need to be done if you really want to be on the ball. June, July, August are ideal times to do your rotations if you are looking for ED LOR and want to apply early.
If you want to barely skim by, Sep and maybe even Oct are the latest to do an ED rotation for a LOR. Even then, there is no gaurantee that the doc you request a LOR from will even send it in on time. Most are procrastinators. Most programs start interviewing in OCT, NOV through JAN.
If you just want to display yourself at a particular ER program and your application is already complete, then you can do this anytime even until January. However, don't schedule to do an ER rotation in DEC and JAN and end up not even being invited for an interview.
For all practical purposes, what Isserson says is correct. Get them done early. The LOR for most MSIV are the biggest rate limiter. Be sure to collect as many of LOR ahead of time as possible.
