When do you tell your back-story for an American Idol audition?

I'm auditioning for American Idol in a few weeks, and I need to know
something:

When do you tell the Idol producers/executives your back-story (if you
have one)? Is it at the very first audition when you're in front of
the table of 4, or is it at a later round? I'm really curious because
a back-story is what helps in getting through, so I'd like to know
when it'll come up so I'll know when I'll have to have everything
ready to say.

Thanks!