It is beautiful because of the autumn's colors but it is quite cold if you want to do a picnic or stay for a long period of time. I will say summer is the best time
Summer and early fall is probably the best time as the weather is warmer for spending long day outside. One thing the senior curator mentioned is the fact that when the ground is dry, you can lay down under some of the art work and get a very different perspective; which would be a bit less comfortable when the ground is wet. Fall is also the time when all the trees in the surrounding area changes colours. On the other hand, with the leaves coming of the trees, it might also be a very interesting view, just make sure you dress warm and for the wind.
It may get cold . We visited in October and the weather was cool and comfortable
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