The Bookday comes as a part of celebration for the most intelligent minds, during this event a quest will appear when you login and will inform you that Bookday event is open. Go talk with Soon for more informations, he will help you from there.
Usage: Place a normal skill book on Soon to recive a tome skill book.
You can read a normal skill book & a tome skill book at the same time and in the same day, tome skill books usually increase the chance of reading, unlike normal skill books where you need to use concentrated reading.
There's no limit for how many books you can exchange, unless the event is over.
Update: You don't need to separate a stack of skill books anymore, before placing them on Soon, this issue was fixed within last patch notes.
Duration of the event: This even is activated/deactivated on a scheduled basis by a Game Master and announced when the event is active.
To find out more events, visit EVENT CALENDAR.