I did an away rotation in June (end of third year).
It usually is required that you are a "fourth year." What that means is that your school must declare you to be a fourth year medical student. Technically my school started the year in July like every other place... but... the lady who does the scheduling in my school said it would be no problem to declare me a fourth year medical student a month early (all other things being equal, and graduation being officially in May anyway, not the end of June).
June was a nice time to do an away as well, since 1) weather was nice, and 2) The CA-1's were almost CA-2's at the time and that gives you a good range of people who are experienced enough for a student to work with.
Kudos to you for thinking of it now, though. If you get your materials together now, you'll get your applications in long before other people are starting to think about and apply for aways, and you're almost certain to get your first choice. I was downloading and preparing my applications around this time last year and sent them in by their respective March deadlines, and got exactly what I applied for with no one saying "We have no more room."