Go get em tiger :-)
Try to remember that you've earned the authority to give that seminar (and it wasn't easy, right?). You have valuable things to teach them, and they're not out to get you. They'll automatically assume you're qualified (which you are) so TRY to relax a little. In terms of academia, you're their superior so there really is no need to be scared of them. Remember first impressions are all important, so plan for a very strong start - try to fake confidence for literally a few minutes, they'll think you're amazing, and it it'll all flow from there.
I'm wasting my time of course - it's normal to be nervous. But just think, in 24 hours you will have done it, and you will not be as nervous (if at all) next time.
Good luck (up)