Unfortunately, we usually have only 1 and a half summers off.
In the one between 1st and 2nd year, people did a variety of things including research, preceptorship (working in a clinic or hospital), traveling, relaxing. This break is usually 2-3 months long.
In the summer between 2nd and 3rd year (1-2 months) most people studied for the USMLE Step II for a chunk of it then relaxed the rest of the time.
There is no summer vacation after third year, you work straight through.