"Miss April-December" is the nickname of the ghost of a human woman.
One of the "Happy Haunts" residing in the Haunted Mansion, Miss April-December's ghost dwells in one of the Changing Portraits (and, simultaneously, as one of the Sinister Eleven). Miss April-December apparently keeps a diary, pictured with her in the portrait, of which she is rather fond.
She draws her nickname from the fact that her haunted portrait shifts between an "April" phase (showing her beautiful youth) to a "December" phase (showing her as an ugly, bitter crone). Her portrait also has "June" and "September" phases, though they are not seen by mortals during tours.
In 2005, her Changing Portrait vanished, though she still remained present in the Manse as her aged Sinister Eleven self ("December").
Behind the scenes
Miss April-December first appeared as a part of the Haunted Mansion attraction in Disneyland, which opened in 1969.
On the fansite Better Haunts & Graveyards, of which Miss April-December is the purported author, she is dubbed Narcissa Devane Gracey, with her dates given as 1840-1922. She does not have an entry in the Ghost Gallery, though some conflate her with "Lillian O'Malley", the Tightrope Girl, due to a certain resemblance.