According to the Aaugh Blog Fantagraphic books will print a comic book called The Unseen Peanuts which as the name denote features over a 100 strips that were unprinted before their inclusion into The Complete Peanuts collection (also published by Fantagraphics).
According to the Aaugh blog author:
Not every store will have The Unseen Peanuts. There are certain comics that shops are required to have on hand to be recognized as a Free Comic Book Day location, but The Unseen Peanuts is not among them.
Those stores that have it won’t have unlimited quantities. Get there early.
You won’t be able to grab as many copies as your little fists can hold. Different stores have different policies about Free Comic Book Day. Some let you take only one comic book, some let you take a bunch… but generally not more than one of the same comic.