Much of the humour in A Christmas Carol comes from the character of Scrooge. In stave one, for example, Scrooge has a conversation with the ghost of Marley, his deceased business partner, which is full of humorous remarks:

"You must have been very slow about it, Jacob,'' Scrooge observed, in a business-like manner, though with humility and deference.
"Slow!'' the Ghost repeated.
"Seven years dead,'' mused Scrooge. "And travelling all the time?''
"The whole time,'' said the Ghost. "No rest, no peace. Incessant torture of remorse.''
"You travel fast?'' said Scrooge.

As we can see, Scrooge's humour reflects his cold and bitter personality: he is sarcastic and scathing in his remarks. But, by the end of the story, Scrooge's sense of humour has undergone an important change. In stave five, for example, Scrooge decides to buy a prize turkey and send it to Bob Cratchit's house. Here, we see another example of Scrooge's humour:

"I'll send it to Bob Cratchit's!'' whispered Scrooge, rubbing his hands, and splitting with a laugh. "He sha'n't know who sends it. It's twice the size of Tiny Tim. Joe Miller never made such a joke as sending it to Bob's will be!"

Later on, Scrooge plays another joke on Bob Cratchit. He pretends to be mad at him for having Christmas Day off and implies that he is about to be fired. But, instead, Scrooge gives Bob a pay rise which prompts much joy and excitement. What we see, then, is that Scrooge's sense of humour has changed quite dramatically. After being visited by the three ghosts, Scrooge's sense of humour is far more sympathetic. He plays (kind) jokes on people and likes to share in their laughter, instead of laughing at their expense.