Take it this summer, as you will regret that the fact that you have study the DAT and your classes simultaneously if you are planning to take it during May.
I took my DAT May 10th, and it was a hellish experience studying for classes and the DAT at the same time.
I highly recommend you get it out of the way this summer so you will be completely ready by the time that AADSAS opens up next year so you can be an early applicant.
If you are planning to take a year off, though, you can wait until next summer, I guess.
But just apply 3rd year so you can chillax and get some B's in your senior year