Some colleges require two or three subject tests, in addition to the SAT or ACT. These are one-hour tests and three can be taken on one day, on the same national test dates for SAT (except March).
Students should check the requirements for their chosen colleges. Generally, excellent students who will apply to some competitive colleges should take 3 Subject Tests in May or June of the junior year. This guarantees that you are prepared for any college you may find interesting!
You can take subject tests in any subject of your choice. If you will declare a major on your application for admission, it makes sense to align the subject tests with that major. For example, Pre-Med applicants would want to take at least 2 science subject tests and Math Level 2.
Some colleges accept the ACT with Writing in place of Subject Tests, so be sure to check on the websites of your chosen colleges for requirements.
Sophomores typically take the Bio Subject Test if they have taken AP Bio. You can take a subject test during any year of high school, at the time that you complete your highest level in a subject area, prior to November of the senior year.