Day 11: First splatbook you begged your DM to approve.
The definition of splatbook can vary depending on who is doing the talking. Most people will quickly think of the Complete series of sourcebooks from 2E. However, I consider anything that offers up new options for characters as a splatbook and thus, I would have to go with the original 1E Unearthed Arcana as the first splatbook. (An argument could be made that such books as Eldritch Wizardry could be considered among the earliest splatbooks.) It also is the book I first wanted to use as a player for my characters. The cavalier class looked awesome. I think it was also the book that showed me that add-ons and supplemental material for the players was okay to use.