What grade is RSI?

What year are you supposed to apply to RSI? Or can you apply at any year, and if so, how many times can you apply? If you know please answer and send the source of your info. Thanks a ton

https://www.cee.org/apply-rsi, http://math.mit.edu/research/highschool/rsi/index.php
RSI is for high school students the summer between junior and senior year, so you apply during junior year.