You can locate Georgetown's multi-year academic calendar here (this will help you, among other things, plan for your graduation date!). The academic calendar is subject to change, so please pay close attention to your email for any shifts in important dates.
The Registrar's Office also has a calendar that details refund and withdrawal dates for all classes. It is critical that you be aware of these dates so you don't risk needing to pay for a course you withdraw from too late.