Actually, there are often more cases during breaks especially orthognathic (school's out, work's out, etc.).
Those are all good places to extern. If you are looking for a lot of experience, sometimes July can be a slow month for the extern because that is when all the new interns arrive for many programs (all depends on the program though and how it's structured). You may want to call beforehand and ask when a good time to come is.
There is never a bad time to do an externship. My program gave interviews to everyone that did an externship (as long as they met the minimum required scores). It's all about getting your face out there. Just make sure you read some old posts about how to be a decent extern.