My husband & I travel up to the Lake a few times every year and have found mid-September to be the best time. We love this time of the year the best because the pace slows a bit -- after the summer crowds and before the ski season begins.
When we arrive in September, we take one full day to visit the Mansion. The grounds are beautiful and quiet. There are times they still hold the tours so you can go into the home.
You can actually take time to slow down, breath deep and enjoy. Best of all is the VIEW from the grounds. From the Mansion's front door/porch, you can enjoy an uninterrupted view of this pristine lake and the surrounding mountains.
Remember to bring your tennis racket, as the courts are open to the public. Not many times "normal" folks can say they played at a Mansion.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.