there is no substitute for spending some time cutting trees throughout the year. I find in this area(ontario) that I can cut hardwoods if the leaves are full and green or no leaves at all(winter or summer).
softwoods are best in the winter, but I've cut lots of softwood in the summer, just have to deal with the sap. Probably the worst time to cut any tree is in the spring, fall is better but you have to watch because the sap is travelling down to the roots and things can get quite sticky in the fall as well.