A Power Seller is someone who has gross sales of $2,000 or more per month. You can't apply for it, eBay monitors your account and offers it to you. You get a few perks but, as I understand it, no reduction in rates.

As far as no-fee days, I've always just found out about them a day or two befor hand on the ebay book board. Don't really know how far in advance they publicize them.
Cheers,
Russ