In general, the summer between your first and second year is free. Most schools start sometime in late July to late August and classes will end around the first week of May.
The second year generally ends around the last week in April or first week in May but USMLE/COMLEX Step I is usually taken during the summer between second and third year. You may be done with class but you are not done with studying. The average US medical student will spend between four and six weeks preparing for Step I.
Third year usually starts in late July early August and is 12-months long. Most students are given two to 4 weeks vacation/break at the end of third year or sometime during fourth year. Of course, there are electives during fourth year that are less busy that others and rotations that are less busy than others.
Many students will elect to complete Step II after the Match or like me (took complete vacation) from Match to graduation as I took Step II early in my fourth year. Graduation is usually in May and then move to wherever you matched to start PGY-1 by late June to early July.
Other than the summer between your first and second year, there is something to do. If you have to remediate anything, remediation classes are conducted between first and second year summer.