If you didn't already know the DAT is available any time of year. In order to take the DAT it is required that you register on the ADA website around a month before you wish to take the exam. Once registered you have one year to take the DAT.
The application for dental school that is submitted through AADSAS will become available around mid-May, this past year it became available May 18th. If you will not be ready to take the DAT by May you can submit your application when it becomes available and your DAT scores will be forwarded to the schools that you have applied to shortly after you complete the DAT. Most feel that it is advantageous to submit your application as early as is possible in your individual situation as it can take up to two months for AADSAS to forward your application to schools once you have submitted it. One advantage of submitting your application even before you take your DAT is that by doing so you can have your applications complete at all of your schools minus your DAT scores. Some schools will require you to fill out secondaries, send pictures, submit appplications fees, etc. in order for your file to be considered complete, all of which can be completed while preparing for your DAT. Once you take your DAT and the scores are forwarded to the schools your applications are complete and you can just sit back and wait for your interviews.
As for Ec's while there is no absolute right or wrong, community service, volunteer activities, and shadowing dental professionals seem to be the most common. Some schools have hour requirements for shadowing so you should really look at the schools you plan on applying to for more detailed information in this area. Even if you apply to schools that do not have shadowing requirements I feel that it is time well spend that can help in answering interview questions such as: what expereinces have you had with dentistry, why dentistry, what aspects of dentsitry interest you the most, etc. that can possibly be answered from knowledge gained from shadowing.
Best of luck!