You can prune the tree while it is dormant in winter or in summer.

Mar 08, When to Prune Fruit Trees The best time for pruning fruit trees is at planting and in subsequent years, in early spring before buds break and trees are still dormant. Pruning should be undertaken at planting time where you cut the new stem off 24 to 30 inches ( cm.) from the ground and remove any side shoots.
Apr 13, Why You Should Prune Plum Trees. There are many reasons to prune plum trees, and the relevance of these reasons will depend on factors such as the age of your tree and the time of the year. Pruning in early spring stimulates new growth. You prune in summer to restrict the size of the plant and open up its interior (canopy).Estimated Reading Time: 8 mins.
Aug 07, The Washington Tree Fruit Research and Extension Center in Wenatchee recommends that you prune very lightly for the first five years. Prune mature trees more heavily, especially if they’ve shown little growth, when all danger from fall or early winter freeze has passed, but before full.