It's a two day annual concert which takes place during the summer. The first two concerts were in July, while the last in 2010 was in August. After New Year there will be official info on their website about the next Miles High Music Festival for 2011.
According to their website, one must check back after new years 2010 for next years dates, but following 2010 festival dates, the best guess is Saturday, August 13th and Sunday, August 14th 2011.
The Mile High Music Festival already took place this year, in August of 2010. The next Mile High Music Festival will be held next year, probably right around the same two days that the festival took place this year.