Sour Cherries can be any one of several cherry varities including Montmorency, Morello, and Early Richmond. According to whatscooking.net,"Montmorency is the most popular of the sour cherry varieties the U.S. and Canada providing 95% or more of the sour cherries on the market."
Prior to this year I was unaware of the distinction between sour cherries and sweet cherries. We grabbed these on a whim at the Lagier Ranches stand at the Grand Lake Farmer's Market. Dumb luck tends to be its own reward at times like these. I did not have to suffer through, for instance, a period of deep and intense wanting.
Sour cherries are smaller than more familiar varieties, and get sweeter the longer you hang on to them. Their tart flavors stand up to the sugary rigors of desserts. While harvest of these cherries is generally associated with July, the folks from Lagier Ranches indicated that last weekend was the last weekend they'd have them.