Street date was August 15th, but some stores (EB Games in Canada) may have started selling copies before then.
Putting the information into Description would be OK, to say the game was to release on August 15 and Best Buy didn't sell it until August 21.
In any case, putting dates into description, don't forget to put the year!
It would be interesting if Best Buy has a different game altogether, such as an exclusive version they sell, then if this is the case then the information wouldn't be valid on the main entry.