Why does Fred say, "I'll keep my Christmas humour to the last"?

What Fred actually says is the following:

I am sorry, with all my heart, to find you so resolute. We have never had any quarrel, to which I have been a party. But I have made the trial in homage to Christmas, and I’ll keep my Christmas humour to the last. So A Merry Christmas, uncle!

This statement follows Fred's invitation to Scrooge early in the story. Fred comes around the day before Christmas to invite his uncle to dine at his house on Christmas Day. Scrooge responds in a very hostile way to the invitation, telling his nephew that Christmas is a waste of time and that it has never done his nephew any good. Scrooge also opines that people who say Merry Christmas should be boiled in their own Christmas puddings.
Fred argues forcefully for the worth of Christmas as a holiday that brings people together in a feeling of common humanity. He says he doesn't care that there is no monetary profit in it.
When Scrooge persists in being extremely rude to Fred, Fred says, in the quote above, that he is sorry that Scrooge is so determined to avoid his Christmas dinner. He implies there is no reason for the refusal, as he and Scrooge have never quarreled. Then he says he has done his part to reach out and will not let Scrooge bring him down. All of this shows that Scrooge's misery cannot penetrate his nephew's happiness.