I recommend taking it Summer of 2019, because it seems you will have done most of the important prereqs such as Ochem, and physiology. Biochem isn't on the DAT anyways so it's not that big of a deal to finish, but O chem and physio will definetly help. In addition, if you have to time to fit in your schedule classes that help for the DAT biology section are genetics, evolution, and developmental biology. Because you are applying summer of junior year also, I recommend taking it mid/late June and giving yourself atleast 8 weeks to do so. It's not crucial anymore to have everything submitted first week of June due to no batches; mid July is fine for most schools if your DAT score is high.