it depends.. do u wanna go to dental school straight from undergrad, or do u wanna take a year or so off?
if you wanna go straight from college, i would take the dat june-aug after your jr year. thats when u would need to submit your application too.
if you plan to take time off after graduation, i would take it in the summer after you graduate and again apply at the same time.
i know some schools send out invitations for interviews in sept. and have interviews oct-dec so i think taking it your senior year might be a little late if you plan on starting dental school after grad.
hope this helps