It's helpful to look at old subscription prices when trying to set new subscription prices. Note that MIT is closed on Christmas Day/Eve and New Year's Day/Eve, and so we don't dance then and the subscription prices should reflect that. Also students often go away between finals and IAP and during spring break, and the student rates should probably reflect that.
Class members get their first class free, so to be sure there's a financial incentive for class members to buy a subscription, the subscription period starts the second week of class and finishes the last week of class. This exactly determines the dates of all of the subscription periods. The dates for class are determined by the PE calendar which is related to the academic calendar.