To sum up December 2015's weather in one word - "WOW"!
As you may recall, November 2015 was New York's mildest on record, edging out November 2001 by 0.1 degree. In December, 2015 once again beat out 2001 as mildest ever, but this time it was by a wide margin - 6.7 degrees. Never before has a warmest/coldest month been that far ahead of the second ranked month. (The previous greatest margin between the top two was 2.8 degrees, February 1934 vs. 1899.) December 2015 was consistently mild throughout the month and was 13.3 degrees above average - the most above average of any month on record. In fact, it wasn't much cooler than the record-mild November and was 3.1 degrees milder than a typical November. Every day of the month had an above average mean daily temperature - the only month in which this has ever occurred. The chilliest day was 12/19, when the mean temperature was two degrees above average (high/low of 40°/35°).
No day in December had a temperature of 32° or colder - a first for December (the coldest reading was 34°). Previously, the mildest daily minimum temperature in December was 28°. 18 days had mean temperatures that were 10+ degrees above average; 13 were 15+ degrees above average, and seven were 20+ degrees above average. There was one streak of nine days with mean temperatures 10+ degrees above average - only a ten-day streak in December 1998 was longer. 11 days had highs of 60° or warmer, another December record. The high of 72° on 12/24 was the tenth time NYC had a high in the 70s in December. And the low that day was the mildest ever reported in December, 63° (13 degrees above that day's previous record). Christmas Eve's mean temperature was 33 degrees above average - the most above average day on record. All told, the month saw four new record highs (and one tied a record) and five record-high minimums (and two ties).
December was the third month in 2015 that was warmest ever. Additionally, May and August were second and third warmest, respectively. These warm months almost completely overshadowed February, which was the second coldest since 1900. (Remember the frozen fountain in Bryant Park, or the ice-clogged Hudson River at month's end?)