What Exactly are SAT IIs and When Should I Start Preparing For Them?

[lkg4answers]

Since you mentioned UCSD in another thread, UCSD’s website says,

For prospective engineering, biological or physical sciences majors:

We recommend that you submit scores from 2 SAT Subject Tests closely related to your major. If you are taking a math subject test, Math Level 2 is required.

If you have your eyes set on UCSD, take an AP science and the corresponding subject test in May or June (after you have taken the course) of your junior year.

https://admissions.ucsd.edu/first-year/application-requirements.html