"I am not sure yet when sumac berries get ripe, but I will find out."

From an article in Countryside & Small Stock Journal, "Sumac, the Wild Lemonade Berry" at http://www.countrysidemag.com/issues/87/87-4/Sam_Thayer.html ...

"You want to get the berries when they are dark red and fully mature, so that they have fully developed their tart flavor, but before the rain has had the opportunity to wash the flavor out. In most of North America, the first clusters are ready to be plucked sometime in July, with the prime time being in early August. Taste each cluster as you harvest to assure yourself that you are collecting something with flavor since occasionally they are bland. A dark purple coloration usually indicates that the flavor of the fruit has developed fully; yet some of the best clusters I've tasted were light pink. Sometimes a white, sticky substance coats the berry heads; this is pure essence of sumac flavor-don't let it scare you off. I pluck about six to eight average-sized clusters for a pitcher of sumac-ade."