Class Day is a school-specific ceremony during which individual students are recognized for their achievements. As their names are read, graduates process across the stage, are photographed, and receive their class pins. Seating is general admission and no tickets are required for guests.
Commencement is a ceremony in which all undergraduate and graduate schools of Columbia University participate. No individual students are recognized, and President Bollinger officially confers degrees to the students en masse. Seating is limited (tickets required), and each student may request tickets when registering (online) to attend.
Gradzone has detailed descriptions of these events including dates and times, as well as specific resources for families.