When I was a kid, I spent a lot of time at the library. Like, a lot of time. The kind librarians gave me a volunteer "job" eventually when they realized I basically lived there. So I guess you could say every day for me was Book Day.
Which is probably why Book Day is, I think, the first made-up holiday I've ever devised. I'm working on a story right now -- kind of Mandalorianish in that it's a "lone wolf and cub," only with a cyborg Terminator-type dude thrown into the mix for fun -- where the hero is caregiver to a very special child, and once a month the near-future rogue librarians of Ferry County prepare a parcel of old-timey who-even-has-those-anymore paper books for kiddo to read and commit to memory. Kiddo looks forward to those books with every fiber of her eight-year-old self. Book Day is the best day.
The North Central Regional Library system in Washington State currently has a mail-order book delivery system in place, which is where the seed of this idea came from. I've just added some futuristic, post-apocalyptic tinsel to the festivity of it all.
This holiday season, may all your Book Days be merry and bright.