I thought National Comic Book Day in USA was in May, when participating comic book shops give away special free giveaway comic books from all the major currently operating US comic book publishers. I've never attended, because I've never been in USA in May except theses last couple years, because of The pandemic. In The Netherlands, in early September, we usually have "Stripdagen", which is a comic book convention held at a major comic book shop or public assembly hall or building (rotating to different parts of the country in different years), where all of the major Dutch, and many of the other European countries' major publishers have representative stalls (desked kiosks), where they show new series, or have artists drawing autographed drawings, and comic book sellers sell current books, and collectors' books and artwork. I guess that is similar to US ComicCons. And there are other comic book conventions during Spring and Summer.
Maybe some American here, can explain to us the difference between the May and September US National "Comic Book Day", and tell us what these celebrations entail.