You can prune the Corylus Contorted Filbert (also called hazelnut and known by  the common name 'Harry Lauder's Walking Stick. " Harry was an old English comedienne who used a twisted branch as a cane)   in fall or spring and I have also pruned mine in the middle of summer to keep the suckers under control. During one hard winter an ice storm caused one third of the branches of my own filbert to break from the tree. The tree survived nicely so I am confident that this interesting specimen plant is not very picky about pruning time.  The contorted version is  a grafted shrub so to turn it into a tree shape you will need to keep cutting back any suckers that sprout from the base of the plant. You can tell the suckers as they grow more straight than the contorted or crooked branches that give this plant its' distinctive form and name. To shape a huge specimen plan on removing one third of the lowest branches each year to gradually expose the gnarled trunk . You can choose two or three of the thickest trunks as the leaders or main trunks rather than trying to create a true tree form with just one trunk. Early spring may be the easiest time to remove the newest sprouts from inside the tree to keep the inner branches more bare to show off their twisty form. Have fun and enjoy the process. You are an artist using  pruning  shears rather than a paintbrush. Keep growing, Marianne Binetti