ChrisAg05 is right. Depending on when you apply, you can go to Jrs after your freshman or sophomore year of college. I think more than half my platoon last summer just finished their freshman year and will not return for Srs until NEXT summer. I completed my sophomore year before attending Jrs so I am returning for Srs THIS summer.
In short, you attend Jrs after completing your freshman or sophomore year of college, but do not return until after you complete your junior year (regardless of when you attended PLC Jrs.)