I prefer harvesting whitewoods(hickory included) in the spring, once the leaves are out. In my way of thinking, in the spring, last years growth ring has time to mature over the winter so I think it is stronger. If you harvest in the late summer or fall you will be using this years new wood. Weather my thinking is correct or not I don't know but harvesting whitewood during the growing season is when the bark slips, making it easier to remove.