I would say anytime but July-August where is is hot and humid. All seasons have their charm - even summer where you can enjoy summer-matsuri with spectacular firework. But you are right about the winter - its quite mild and certainly more pleasant than most places in North America and Europe. When I stay in Japan during Dec-Jan, wich I usually do in order to celebrate New Year with my in-laws, I am always extra pleased that I as an extra bonus, avoid some of the misserable weather we have in Scandinavia during that season.