The "Huh Huh Humbug" segment marks the only time Principal McVicker's family has ever appeared on the show (though since this is intended more as a parody of A Christmas Carol than anything else, and actually happens in a dream, whether he has a big family like this for real is debatable.)
"Huh Huh Humbug" is a parody of the classic Charles Dickens story, A Christmas Carol. Butt-Head's character is the equivalent of Jacob Marley, and the McVicker family is the equivalent of the Crachit family.
During "Huh Huh Humbug" when Beavis returns home from Burger World, he's humming part of "Jingle Bells.", but on all home video releases, it was changed to "Winter Wonderland".