National Portfolio Days are events held around the country where art students can learn about different art programs as well as bring their work to receive feedback on it from representatives of top art schools and colleges. No admissions decisions are offered on National Portfolio Days; instead, they are a way for schools to look at your portfolio-in-progress and give advice. They are an excellent opportunity to get feedback from schools you are considering applying to and to ask them specific questions about their portfolio requirements.
It is a good idea to attend first as a junior and get some early feedback on your work, then attend again as a senior when you have made more progress on your portfolio. You should definitely stop by the tables of schools you want to apply to, but consider stopping by the tables of other schools as well, as a review from any school gives you valuable feedback.