Having done this for 35 years on and off, here are my thoughts: Whenever you have the time, but if you are machine splitting, I do it right away, since it splits easy and it smells good, especially red oak and black birch. If you are talking splitting red oak by hand, I stand the logs on end, wait until it is below freezing, (the lower the better) and then attack with a maul. Frozen wood falls apart when frozen and after a few logs, you are not cold!! But I really like early spring and late fall, when there is nothing else that needs to be done. There is nothing like it.