There are three general rules to follow when picking the date you should take your subject tests:
Don’t cram! These are simple, straightforward tests of long-term memory and subject mastery. These test are made up of knowledge based questions. You can’t learn the entire US History in one month so you shouldn’t be studying for a US History in over the course one month. Find a date that is far enough to allow you to prep sufficiently. Even 15 minutes a day will work.
Do them as early as possible! You shouldn’t leave these test until the last minute. Give yourself time to get great scores. Remember, if your scores aren’t great, a college won’t eliminate you immediately, but if your scores are above the rest, it WILL put you ahead of other applicants.
Take tests in areas where you are already strong or right after you take the class in school! So as soon as you finish Biology and it’s fresh in your mind, sign up to take the test!