This is highly dependent on personal and financial circumstances, but if you have flexibility:

•The latest is December of senior year if you are applying Regular Decision without a gap year
•The latest is October of senior year if you are applying Early Decision/Restrictive Early Action/Early Action without a gap year

•I recommend taking it August or October before junior year
This way, you have time to retake if necessary, and more time in the summer to study since school is off