Spring extension is an extra 4 to 6 weeks of classes/clinics that come after you complete your Spring semester. The classes/clinic actually count towards your fall semester - that is, the classes and clinic work we are doing now as part of Spring Extension actually are part of our Fall 2008 curriculum.
The summer breaks run from early to mid June to about mid August. However, other activities such as board studying, externships, etc. often occur during these breaks.