There was a study done on the March 2012 heatwave and I believe they concluded that it was a 1 in 4,000 year event at the time. I still think it ruined some people’s perceptions of how a normal spring is supposed to look, even to this day.
Then on the flip side you have Feb 2015 which was one of the coldest months in modern history. Not quite as anomalous as March 2012, but still a very impressive month.