It’s been said with good reason that Yellowstone, a park with an elevation of mostly 7,000 to 8,000 feet, has only two seasons, summer and winter. Mid October is the end of the summer one, not just because the aspen leaves have turned golden and snow may begin staying on the ground, but because facilities and then roads are beginning to close. Most lodging is already closed now, and all interior park roads close on November 8th  for the quiet time that lasts until December 15th.
This  was the busiest summer the park has ever seen. More people are learning what a phenomenal place it is. This testimonial I recently read says it well: “Traveling through this huge & magnificent park is something you’ve heard about your entire life. It seems like the hype would be impossible to live up to. Untrue! We spent 2 days and could have easily spent 2 weeks. The views are constantly changing and the geology is like nowhere else on earth,” (http://billgisele.blogspot.com/2010/10/yellowstone-park.html).
Just in time to be closed again, the new road and bridge over the Gibbon River opened to traffic a few days ago. The nearby picnic area is to open on October 23rd, so you have two weeks to set up your picnic there before all the roads are closed!