All exams are on Friday except during the Neuro block where you have an exam every Monday. Exams are usually every 3-4 weeks.
For instance, for first year, we had 4 exams during anatomy, 3 exams during MCBM, and 4 exams for physio/histo. Anatomy and MCBM also have quizzes in between exams. For second year, you do only have one exam each block once you start the organ systems. You have to pass every block exam to pass the course.
Then, you have ICM (basically Doctoring 101) exams in between those. They are generally very easy, and people just cram the night/hours before. No real studying for these.
EDIT:
http://www.jefferson.edu/jmc/admissions/Academic_Calendar.cfm You can see the exam schedule for first year here.