Hi usually suggested good time for Tawang is April May June or for winter lover Sep-Oct. From Mid November until January end usually people avoid due to severe snowfall. But many people still plan during December to experience extreme snow conditions & views. so if like to experience heavy snow can plan with proper precautions as well checking weather conditions & availability of transportation etc . FYI- I visited during February roads were clear and there were snow on peaks but not on roads until sela pass but after that there were modarate snow on road too, was facing difficulty in riding Hope above information will help in planning your trip Thank You
See You on the Road...👌👌
Ofcourse December is the time to vist arunachal pradesh as what fun will it be there if you visit arunachal pradesh and u don't feel cold, so brother yes December is the time