The general time of the conference should be suggested by the organization you are hosting (e.g., your regional group). Then look at your campus schedule to determine what dates would be best. Submit a couple of possibilities to your organization for approval.
Consider the size and scope of your conference. Small groups can be hosted nearly any time. But larger groups will require housing, transportation, and food services that might conflict with other campus events. Summers and between semesters/quarters are often better than when school is in session. Avoid football and parent weekends if possible. Check for other major events that might conflict.