Jeez! I leave for awhile and everything goes pear shaped!

Here's what I was taught in Catholic school:
All Hallows is an old way of saying All Saints
(Hallowed-Holy)
All Hallows Eve is the evening before All Saints Day on Nov. 1st - hence October 31st - All Hallows Eve - becomes Halloween.
Even OED agrees with me ... those clever devils!
As for The Day of the Dead - here's what I Googled:
The original celebration can be traced to the festivities held during the Aztec month of Miccailhuitontli, ritually presided by the goddess Mictecacihuatl ("Lady of the Dead"), and dedicated to children and the dead. The rituals during this month also featured a festivity dedicated to the major Aztec war deity, Huitzilopochtli ("Sinister Hummingbird"). In the Aztec calendar, this ritual fell roughly at the end of the Gregorian month of July and the beginning of August, but in the postconquest era it was moved by Spanish priests so that it coincided with the Christian holiday of All Hallows Eve (in Spanish: "Día de Todos Santos,") in a vain effort to transform this from a "profane" to a Christian celebration. The result is that Mexicans now celebrate the day of the dead during the first two days of November, rather than at the beginning of summer, but remember the dead they still do, and the modern festivity is characterized by the traditional Mexican blend of ancient aboriginal and introduced Christian features.
Generalizing broadly, the day's activities consist of visits by families to the graves of their close kin. At the gravesites family members engage in sprucing up the gravesite, decorating it with flowers, setting out and enjoying a picnic, and interacting socially with other family and community members who gather at the cemetary. Families remember the departed by telling stories about them. The meals prepared for these picnics are sumptuous, usually featuring meat dishes in spicy sauces, a special egg-batter bread, cookies, chocolate, and sugary confections in a variety of animal or skull shapes. Gravesites or family altars are profusely decorated with flowers (primarily large, bright flowers such as marigolds and crysanthemums), and adorned with religious amulets and (in smaller villages) with offerings of food, cigarettes and alcoholic beverages. Because of this warm social environment, the colorful setting, and the abundance of food, drink and good company this conmmemoration of the dead has pleasant overtones for most observers, in spite of the open fatalism exhibited by all participants, whose festive interaction with living and dead in an important social ritual is a way of recognizing the cycle of life and death that is human existance.
You can read the whole thing at http://
www.public.iastate.edu/~rjsalvad/scmfaq/muertos.html
Sounds like an exuberant version of our Memorial Day!