Best time to take your 1st DAT would be between spring-summer, and between summer-fall for the 2nd (you have to anyways, because of the 90 days re-take period). However, you can take it any time you like, it's just getting the scores sooner that counts, so schools can evaluate your file sooner.
Also, there are some schools that won't accept DAT scores that were taken after a certain date. I remember hearing of some with deadlines of the latest you can take the DAT for that application cycle. Some were as early as December while others were in March. Basically, it's a good idea to check that out at each individual school if you are planning on waiting to take it. Remember though that many schools will not even look at your application without your DAT scores, so try to take it early.