What month would you recommend to visit during the summer?
RMNP is busy throughout the Summer season, starting from opening through Labor Day. I mentioned weather as a factor in my previous post, so you can be aware of the changeable nature of it up there. Whereas in a normal year, NPS gets Trail Ridge open generally in mid-May, this year it was after Memorial Day, which is unusual.
That said, I would look to perhaps some time in July as the optimal time, followed by August. Those 2 months there's less of a chance of snow at the higher elevations, though it can occur any time of the year up there. Yes, those 2 months will be busy with a lot of people, but there's a reason for that -- weather is generally the most ideal for exploring Rocky. July is a bit drier than May or June, and the monsoon season doesn't kick in until the end of the month/early August. August can be wet, but it's generally rain and generally thunderstorms that build up in the afternoon. Rule of thumb for the mountains is to get an early start and be off the high peaks by 2pm at the latest.
September is also a good time to be there as well, but Glacier Basin closes in early September, so there's more competition for spots in Moraine Park. September/October is the rutting season for the Elk and they are everywhere and quite a spectacle. Day users of the Park are also everywhere and traffic can get crazy some days/evenings.
Make your reservations in January of 2016, February at the latest, depending on when you choose. If you want to know about some specific sites and what your needs might be, PM me and I'll try to assist.