Do I have to take the SAT Subject Test and when?

I know the SAT is a mandatory test that colleges look at, but I’m confused if I should take the SAT Subject Test. Yes, I do know the subject tests are necessary for certain majors and certain colleges, but for me, I may be an “Undeclared” major in college so do I still have to take it? Also, if I change my undeclared major to like a biology major, then do I still have to take the subject tests?

Also, is it necessary to take the subject tests during the time frame where it’s good the take the regular SAT (Mid-junior/senior year) or just take the SAT during anytime in Senior year?

Sorry I’m very confused

If the college does not require/recommend Subject Tests, then no, they are not necessary. If a college says they will consider your scores, then you might want to consider them. Major generally does not matter - a college will state its preference for all applicants.

As to when to take, it depends upon the test. However, IMO, it’s best to take in either May or June of the year you are doing the subject.

There are eight colleges in the nation left that still actually require subject tests of most applicants – Caltech, Carnegie Mellon, Cornell, Harvey Mudd, MIT, Rice, Tufts and Webb – and those require them even if you are undecided. Harvard, whose policy is schizophrenic, in that it states that subject tests are “normally required” but that you can choose not to submit them and still be considered for admission, applies whatever that policy actually is to all applicants, undecided or otherwise. There are then about two dozen colleges that recommend subject tests and most of those recommend them for most applicants, including undecided, but some of those limit the recommendation to particular schools within the university such as engineering. There are then another two dozen or more colleges that neither require nor recommend subject tests but will consider them if submitted and those consider them regardless of major including if undecided.

You have to plan carefully because you cannot take subject tests at the same date as the 'regular ’ sat. Therefore, you should take subject tests right after you end school and practice with a book of tests from your public library, then prep for the sat itself and take it in June.
Any two or three will do - English is a relatively easy one, then either math2+a science or foreign language + history, or, if you’re truly undecided, any combination.
In the Fall, you retake the subject test you scored the lowest on and one more - this covers you for all colleges.
Some colleges, called 'test flexible ', will also consider one humanities and one math test plus one of your choice, as equivalent to the sat. So if you can score 750 in English and French and 600 in math 2, you’d be good because they’d consider it the same Dayna’s if you’d scored 2100 on the old sat.
You also retake the sat at least once between August and December.
Most scholarships rest on scores so it’s a good use if your time to practice a lot. (Use Khan academy).