The public oral defense of a thesis is intended to be an examination of a completed body of work and should be scheduled only when the dissertation is essentially completed. The General Announcements is the authority on all things related to candidacy, defense and thesis submission. Information for doctoral degrees can be found here, and master's degrees can be found here. Your graduate program may have additional requirements.
Students may also consult the last semester timeline for detailed information on the defense process, as well as the thesis defense instructions here. Forms related to candidacy and defense can be found here.
*Individual time boundaries for candidacy and defense can be found in Esther