I should have a look at the history of this flight as the dates on the cover were overlooked by me because the sheer delight of ownership took precedence
The launch was originally scheduled for August 14th but I'm not sure what prompted the delay (most likely weather).
Like you, the thought of owning a cover that had flown in space was appealing to me; enough so that I purchased 3 from USPS. It took on even more meaning when Challenger and its crew were lost 73 seconds after launch on January 28, 1986.
There were several firsts associated with the 8th shuttle flight (and Challenger's third): it was the first night launch, the first night landing, and Mission Specialist Bluford was the first African-American to fly in space.
This link is to the press kit issued for the flight -- page 37 talks about the covers. https://docs.google.com/viewer?a=v&...e3rgFSIc6NdA