What's the best time of the year to visit there??

The temperature is usually about 10 degrees lower from the rest of AZ. I would say anytime besides the dead of summer is a great choice!

First week of may is perfect.

Depends on what you want to do, and what type of weather you enjoy. It was very hot when I was there in June. I have been told it is beautiful in the winter too.

I went in mid September and it was awesome! It wasn't too hot and the tree's were still green. I would think that spring time would be nice as well!