What's the best time to go to Sapporo in Japan? Here are some annual weather facts we collected from our historical weather data: The months July, August and September have nice weather with a good average temperature. Most rainfall (rainy season) is seen in May, June, July, August, September and October. Cold season / winter is in the months January and February. On average, the warmest month is August. On average, the coolest month is January.