I have found that the most ideal time to prune grapes is February, when the branches have become supple enough to be bent a little. There are lots of different ways to train grapes, but you should know that some varieties will not produce flowers on shoots that emerge from the first couple of buds of a branch. With these, you need to choose a training style that will allow you to replace whole branch lengths every year. With other types, you can establish a form and prune last summer's stems back to a few buds all along that form. Does that make sense? I'm pretty sure that Concord grapes will not flower at the base of last year's growth, and that Candace is a type that will. If this makes no sense, let me know, and I will try to explain it better.