Well, doing Skyline in winter snow is simply very hazardous because of the steep slope along the traverse at 8000'. There have been a number of deaths there in icy conditions, including a professional mountain guide a few years back. Getting into May and June...frankly, I don't think anybody pays attention to it. May is essentially the only good month in the spring to do C2C. But if you have your choice of times of year to do C2C, fall is easier than spring anyway, and this would be another good reason to choose fall.