Best months for away rotations are July, August, and September. Doing it then gives enough time that your grade/evaluation should be available in time to be included in your Dean's Letter, and also allows enough time for your attending at the away rotation to write you an LOR. Doing it later means you probably won't be able to make it "count" in your official ERAS application.
On the other hand, I ended up doing an away rotation at a program I was very interested in during November due to scheduling problems (had intended to do it during the summer). It was still a great experience from my end - getting to see how things work in a different hospital, different region of the US, see what life would be like in that city, etc. And although it was too late for my attending to write an "official" LOR, he made phone calls to a few places where he had strong connections and wrote a "supplemental LOR" for the residency program at that school, both of which were very helpful in strengthening my application.
In short, I think it's worth doing anytime, but probably most useful in the summer or early fall.
