Question: Who sang the best day of my life?
Disambiguated questions and answers: question: Who sang the 2013 version of Best Day of My Life?
Who sang the March 29th, 2014 version of Best Day of My Life?
Who sang the easy listening version of Best Day of My Life?
answer: American Authors
Tyler Ward and Cimorelli
Vitamin String Quartet
Natural Questions answer: American Authors (from the Wikipedia article "Best Day of My Life")