As far as what will happen that day, The Denver Post has it covered:
While the date of the meeting between parties has been set, the particulars have not yet been formalized. Still to be determined is whether there will be a formal hearing, a preliminary discussion, or whether Miller will be present.
While some form of hearing will be held Aug. 15, a ruling on Miller's appeal is not expected for several more days. A ruling is expected before the Broncos open the season with a Thursday night, Sept. 5 game against the defending Super Bowl champion Baltimore Ravens.
It has already been established that Von Miller DID NOT test positive for a performance-enhancing drug. It has also been established that the possible suspension IS NOT because Von tested positive for an illegal substance since his original positive drug test in 2011. Von tested positive for Marijuana in 2011 and an "undisclosed amphetamine" rumored to be "Molly" (a pure form of the drug "ecstasy") and was subsequently placed into the leagues' substance-abuse program. Miller was not disciplined for the amphetamine because it was not classified as a performance enhancer. The pass-rushing phenom also suffers from asthma and medication stemming from treatment of his asthma could also be the root of the positive amphetamine test.
The four game suspension that is hanging over both Von Miller and the Broncos can be for a number of reasons including failure to show up for a required drug test. Whatever happens on the 15th and thereafter, members of the organization have stood by Von and pledged their support. Miller himself also stated from the beginning that he did nothing wrong.