In my opinion, you'll want to take it earlier than December. Many (or most) schools won't even consider your application complete and ready to review until you've taken the DAT. Many recommend taking the DAT by October at the latest in order to remain competitive.
It's still possible to get in if you take it in December, but it decreases your chances dramatically depending on where you apply. Keep in mind that schools will have already begun accepting students by December 1st.