City	Average Daytime High	Average Nighttime Low	Rainy/Snowy Days Early Month Late Month		Sunny Days Early Month Late Month
Sapporo	2 C (36 F)	-4 C (25 F)	50%	50%	35%	35%
Tokyo	12 C (54 F)	5 C (41 F)	15%	10%	65%	75%
Takayama	6 C (43 F)	-2 C (28 F)	40%	40%	35%	35%
Osaka	12 C (54 F)	5 C (41 F)	20%	15%	65%	65%
Fukuoka	13 C (55 F)	5 C (41 F)	30%	25%	45%	45%
Naha	21 C (70 F)	16 C (61 F)	25%	30%	45%	40%

December is a good month for traveling thanks to generally dry weather conditions. Domestic travel activity remains low during the first half of December until the beginning of winter school holidays around December 23. The downsides of a visit in winter are the relatively short days (sunset is around 4:30pm in Tokyo) and the barren state of the vegetation. From around December 29 some tourist attractions close down for the New Year holidays. The ski season starts in December.

Credits to: Japan-Guide