USC will not send the secondary until you complete your interview. Before you get the interview invite, you have to pass 3 rounds' review. As soon as you pass each round of review, you will receive a notice by mail. For class of 2008, they will send an interview invite through email first and then, mail.
For last year, the first interview dates were 10/10 and 10/13. I got the email around 10/5.