When an unscheduled session is called by the President it is called a special session. Special sessions may be convened to address special topics, or emergencies such as war or natural disaster.

A "special" session is a session of a body that is convened at any time other than that which is scheduled.

Because of the presidents power to call a special session

A special session of Congress is called in the event of an emergency, whereas Congress would normally only have one session a year.