@OhPearl Rules for Harvard cross registration are that you cannot take any Harvard courses in your freshman yea rat MIT. They note that spring break is different at Harvard and the start dates of the semester at MIT and Harvard are different:
http://web.mit.edu/registrar/reg/xreg/ugradMITtoHarvard.html
Taking classes at Wellesley is the best bet for MIT students as there are less restrictions and very good transportation between the two campuses. Its a very old cross registration program that has been very successful for many MIT students. I think the MIT and Wellesley calendars line up better than Harvard/MIT.