A special session of Congress is a session that can be done at any time and is called by the President. It isn't at a specific period in time every year.
As regards a joint session of Congress, it is a gathering of members of the two chambers: the Senate and the House of Representatives, from the bicameral legislature of the government. This session is done on a special occasion but is required to be done with the President.
Moreover, a joint session is ceremonial since it doesn't have any legislative function and is mostly held in the Chamber of the House of Representatives.
