The term 'December Ditty' is composed of four words, each with a different number of letters. The first word is 'NOEL', which is a French word meaning 'CHRISTMAS'. The second word is 'CAROL', which is a traditional CHRISTMAS song or hymn. The third word is 'ARIA', which is an elaborate vocal solo in an opera or other musical work. The fourth word is 'Tra', which is a type of Italian folk song. All of these words are associated with the CHRISTMAS season and can be used to create a festive ditty for December.