See link for add/drop deadlines and pre-enrollment for Fall 2017.

http://www.engineering.cornell.edu/academics/dates-and-deadlines.cfm

Note: if you are earning your B.S. degree in May or August, you will be unable to pre-enroll for Fall 2017.  You will be eligible to enroll when electronic enrollment resumes in Summer 2017.

Please check your current schedule for Spring 2017 to make sure it is accurate.