December, the last month in Gregorian calendar corresponds to Poush or the 9th month of Nepali calendar.
Poush and the succeeding month Magh together is the coldest time of the year in Nepal.
Temperature in the Himalayas drops below 0 degree Celsius and the lakes, waterfalls and rivers there usually turn into ice.
In valleys like Kathmandu and Pokhara, the temperature remains below 5 degree Celsius with surrounding hills occasionally receiving snowfall.
Meanwhile the climate in the southern plains of the country is relatively warmer with balmy temperature of around 10-20 degree Celsius.
Given such considerable difference in climate, you can travel from snow filled Himalayan villages to tourists can be seen playing with snow in the Himalayas in freezing weather while down in southern plains, there would be people enjoying sun bathing and jungle safari inside the national park.
If you love the idea of snow and winter, then December could be best time to travel to Nepal.
Trekking in lower elevation, especially below 3000m, will be an adventure to take on. Cultural tours in Kathmandu Valley and Pokhara, jungle safari in Chitwan National Park or Koshi Tappu Wildlife Reserve or skiing in the Himalayan areas are some of the most popular activities to involve in.