Christmas Idioms 

Christmas carol 

Explanation :
The dictionary definition of carol is a 'joyous song or hymn', but the word carol is almost never heard except in connection with Christmas.  So, a Christmas carol is basically any song meant to be sung at Christmas time.  'A Christmas Carol' is also the title of a famous work by Charles Dickens which tells the story of Ebenezer Scrooge, a tightfisted businessman, and how he learns to celebrate Christmas.  The term 'Scrooge' has since come to mean anyone who is selfish.