Día de los Muertos (Day of the Dead) is an interesting holiday, in that it is a blending of traditional indigenous celebrations and forced association with Catholic holidays. November 1st and 2nd are significant in Catholicism as All Saints' Day (aka Solemnity of All Saints, All Hallows, and Hallowmass) and All Souls' Day (aka Feast of All Souls or The Commemoration of All the Faithful Departed). The original Aztec festival honored the goddess Mictecacihuatl, the Lady of the Dead. When Spanish conquistadores encountered indigenous peoples' celebrations for the dead, they ended the month-long celebration and moved it to coincide with the Catholic holidays in November. Now, Mexicans celebrate Día de los Inocentes (aka Día de los Angelitos; Day of the Innocent/Little Angels) on November 1st and Día de los Muertos (aka Día de los Difuntos; Day of the Dead) on November 2nd.
Prior to these holidays, families will prepare ofrendas, offerings of toys, food, alcohol, and other trinkets for both the graves of the deceased and their own homes. Azerothians will need to visit the graveyard nearest their race's capital city to participate. Once there, families will clean and decorate the grave, then picnic, dance, share humorous stories about the deceased, and even spend the night! Many people will wear skeleton costumes or masks, or even dress up as the dead. It is considered good luck to own something of the departed's. When dancing, some people will attach shells to their clothing, believing that the noise will attract the dead.
[As a quick aside, I took Spanish throughout high school and university, and had to watch a classroom telenovela known as La Catrina. The way the main character said that phrase has always stuck with me, so I find myself hearing it over and over as I read about LA CATRRRRIIIIINAAAAA.]
To get the one achievement associated with the holiday, players must /dance with La Catrina in the graveyard. This will also give them the buff Honor the Dead, which makes them look like a skeleton. Being walking skeletons already, the Forsaken are uniquely suited to represent this holiday. However, it's interesting in that they themselves are denied what those being celebrated have: the blessed afterlife!