When should you take the DAT? Well, _every_single_person_ on this board will tell you to take it as early in the cycle as possible, so you can get your complete application submitted. AADSAS opens May 17, and (I think) you can apply starting 6/1. So take the DAT as soon as possible after the time you feel prepared to get the score you are shooting for. The sooner, the better. The longer you wait, the lower your chances for being accepted. At the same time, you cannot take the DAT until you are prepared for it, so study hard now, so you are prepared to take the DAT earlier rather than later.
Is it too late if you take the DAT in Aug? No, but your chances of being accepted are lower than if you got the same score in June. Is it too late in Oct? No, but your chance are even lower. And Dec? Again, no, but your chance are even lower than low. Get the trend?
Look at the schedule in reverse. It becomes obvious when you need to take the DAT:
Dec 1: 1st day any school can accept you. You want to be part of this batch.
Oct/early Nov: Good months to be interviewed. You're early in the cycle.
Sep 15: Schools grant you interviews for mid Oct.
Aug 15(ish): You complete your secondary app.
Aug 1: Dental schools review all your stuff and send out secondary (if there is one).
July 1: AADSAS sends your stuff out to dental schools.
June 1: submit AADSAS application.
I've padded the schedule a bit, but my point is this: It is very important you have your application complete as close as you can to June 1, if you want to be accepted on Dec 1...at least it will increase your chances greatly. Also, if there's anything incomplete in your application, the earliest the dental school can tell you is in Aug or Sep and it will take a roughly a month to get resolved...like missing LOR. That will push the timeline back even further.
Study hard and good luck always.