December is one of the coldest months of the year in Las Vegas, USA, when the winter season is in full swing. The average temperature at the beginning of the month starts off at 10°C (created by highs of 15°C and lows of 5°C) on December 1, befor... Averages for Las Vegas in December. December is one of the coldest months of the year in Las Vegas, USA, when the winter season is in full swing. The average temperature at the beginning of the month starts off at 10°C (created by highs of 15°C and lows of 5°C) on December 1, befor... Read more. January.