Well I'm a day late on this but I wanted to acknowledge this momentous event. While it may be true that this probably isn't the official date that the band was formed, it was definitely the start.
When I went to the Eagles show in D.C. a few years back, Glenn said that after he and Don joined Linda's band, they practiced a few weeks. and their first shows were in D.C. at a club called the Cellar Door. It was after a show one night sitting at the bar drinking tequila that he and Don definitely decided they wanted to form their own band. At least, that was his recollection on that night.