One man said it was getting towards the long days and the short nights, now. Then they laughed. ch.7 p.33. Why is it a laughing matter?

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.