So, if you're a librarian, consider doing a display at your library branch to celebrate SF/F writers...and if you work in a bookstore, or own one, consider making that day special for SF/F writers and fans. And if you're an SF/F fan, make that the day you go buy a book. And if you're an SF/F writer who isn't on a tight deadline, it might be fun to hang out at your favorite bookstore and be Visible (if your favorite bookstore is using that day to promote the day, they might even provide a coffee...)
I'm expecting the delivery of the copy-edited manuscript that day, and have committed myself to waiting at home for the parcel delivery, but the rest of you can go forth and celebrate. (Digging into the copy-edited ms. is rarely a celebration, as we writers know.)