Sarah SammisOn page 42 we learn that Ozma's birthday is coming up on the 21st of the month. We know from page 15 that it's currently August. If Oz is on the same calendar, then Ozma's birthday is August 21st.
Fox King: "[W]hatever country Dorothy visits is sure to become famous."
Dorothy: "How did your Majesty know my name?"
Fox King: "Why, everybody knows you, my dear. Don't you realize that? You are quite an important personage since Princess Ozma of Oz made you her friend." (p. 41)