Sir Thursday is a young adult fantasy novel written by Australian author Garth Nix. It is the fourth book in The Keys to the Kingdom series, and was released in March 2006. Sir Thursday continues from the preceding book, following the adventures of Arthur Penhaligon as he attempts to retrieve the Fourth key of the Kingdom, and claim mastership of The Great Maze. As with the other books in the series, the theme of 'seven' is prevalent, along with the themes of sin and virtue.