If all else fails, weather.com has reports for most areas, including average and record temperatures for a location. It's a US site, but I've gotten most international locations I needed from it. Just type in the city you need.
I was in Kansai last December. It was mild to cool each day. I don't know if that's typical.
Here is the page for the Osaka airport at wunderground.com: Local Weather Forecast, News and Conditions | Weather Underground
Where it says "Detailed History and Climate," you can pick several December dates from recent years and look at the data. Naturally, you can look at other areas too, if you know what city you'll be in.
50 degrees or warmer? i was in tokyo last year @ dec and all i needed was a windbreaker. with the war going on it feels even warmer
indiv pref may vary?
Really? Last time I was in Tokyo it snowed. Just once, but it snowed...it was damn windy too. It wasn't as cold as up north, but maybe some people are just more used to cold weather...what with me being from LA and all.