I think that you are referring to a line from this book, so I have moved your question to this group. It will be easier for you to get good answers if you specify what work you are talking about.
In Chapter 7, Huck hears the men across the river talking about the long days and short nights. I do not think that the simple statement about the long days and short nights is really actually funny. Instead, I think what Twain is trying to tell us is that those men that Huck hears are drunk.
The reason I say this is because they are laughing at things that aren't really that funny (when the one guy says he figures this is not a short night) and because they keep repeating that "joke." This sounds very much like how some people act when they are drunk. So I think that the men are only laughing at this, that they only think it is funny, because they are drunk.