2 years ago, my niece got married at Crested Butte, Co. in the middle of October at 10,000 feet elevation. It was a gorgeous day with the high in the 70's and sunny and the sky soo blue as only Colorado seems to have! The wedding was outside at 4, I think. When we got that high up, it was chilly, but the view and the wedding was gorgeous with the mountains and Aspens at their prime, I think a dress or a dressy pantsuit would be nice. I saw lots of boots, but it was not black-tie, so I don't think they would be good for your wedding I would take a coat, maybe even a winter coat as the week after she got married, it snowed 20 inches and they were skiing!! I would also advise you to drink lots and lots of water and to take chapstick as the elevation dries you out tremendously. Have fun and enjoy your trip. We all thought it was the best wedding ever!!!