You need to be contracted to apply, and applications are usually due end of October. My Dd did it this year and it was well worth it. She is a rising MS4 and went from CULP straight to advanced camp. So if you were not able to apply until Junior year it is still possible.
The main thing I am aware of if you are not contracted to apply for is Project GO, it is a DoD program for critical languages. The DoD pays for all expenses for the program and some are domestic and others are a study abroad situation. My daughter applied her freshman year and was waitlisted for the program she applied for. She ultimately did not get it, but is something to consider if you are interested in a critical language, possibly studying abroad in the summer and not contracted. Of course contracted cadets can do it also, it just isn’t required,