But why June 21, 2006, specifically? According to Wikipedia, there was a circulated Pre-RC1 Vista build (but only the first of 3) that was built on June 20th, and June 21st was the first day of summer that year, but neither of those seem like good reasons to pick this date in particular.
Much more than that. For example, both audio and video drivers had massive rewrites and rebuilds. Video drivers received a lot of tweaks for DirectX 10. More importantly, MS pushed the entire audio stack to use the User-Mode Driver Framework [1], which resulted in a lot of changes in how audio drivers access the underlying hardware.
