The first Apple store opened on May 19th, in McLean, VA and Glendale, CA. Apple employees have been put on a hold for vacation between May 20th and May 22nd. Apple is still being very secretive about this as well.
Not much is known at this point. Will it be a product launch? Not likely. Many speculations are that the new iPhone will be here in September, and the iMacs may be refreshed later than May. A special promotion? Possibly. All we know is that managers of the Apple stores are very excited about those dates so we can only assume we should be too.