What Christmas song has the lyrics is here bum ba da dum have a merry

"Here we come a-wassailing" is a traditional English Christmas carol that includes the lyrics "here we come a-wassailing among the leaves so green; here we come a-wand'ring so fair to be seen. Love and joy come to you, and to you your wassail too; and God bless you and send you a happy new year. And God send you a happy new year."