National Convention is like State on steroids. It lasts 1 week at the end of July (specific dates vary by year) and takes place on a college campus (which also varies by year). It has Olympika and Ludi (sports like basketball and track and field) in addition to normal convention activities like testing, Certamen, and art. You can also run for National office here, and become one of the NJCL officers.
More about Nationals activities here.