It is best to visit Japan in spring and fall. The climate is varied, but in most parts you will find four seasons. The far north can have very cold winters with winds from Siberia (brrrr) while the far south and Okinawa enjoy a nice tropical climate.
If you’re going to Sapporo by plane, please book in advance because the route to and from Tokyo is very popular.
If you have any food allergies, always clarify them when ordering in restaurants to avoid discomfort.