I am a current Maryland student and the way that the registration process works is they will begin to allow students to register (based on credit and undergrad/grad student, more credits gets priority). You can most certainly register after the 31, and unless you wait until May to register, you should be able to get the classes you need. Also, if you get waitlisted for a course, emailing the professor helps sometimes, and if not advisors are sometimes able to pull strings. I hope this helps and congrats on getting accepted! =)