Before `` Treehouse of Horror XI '' , which aired in 2000 , every episode has aired in the week preceding or on October 31 ; `` Treehouse of Horror II '' and `` Treehouse of Horror X '' are the only episodes to air on Halloween .
Before `` Treehouse of Horror XI '' , which aired in 2000 , every episode was aired in the week preceding or on October 31 ; `` Treehouse of Horror II '' and `` Treehouse of Horror X '' are the only episodes to air on Halloween .