Halloween is actually one of our oldest holidays. The first celebrations were with the Celts in what is now Great Britain, and northern France. The Celts date back to over 2500 years ago. They honored Samhain, the god of the dead. They had a festival for him on Oct. 31 called the Vigil of Samhain. They believed that during this festival spirits of the dead roamed the earth. People wore costumes so the spirits would not recognize them. So, yes, wearing costumes on Halloween came from the Celts. Believe it or not we also get trick-or-treating from them. Farmers used to go to rich homes and ask for food. If they did not get any the farmers would play mean tricks.In early Ireland they carved turnips for Jack-o-lanterns. When they came here they used pumpkins instead.
The Romans also had celebrations around the same time of year. In late October they celebrated The Feralia, a celebration for the dead. In addition on Nov. 1 they celebrated Pomona, the mistress of fruit and gardens. They laid out things for her such as fruits and nuts in thanks for the good harvest. When the Romans came to Britain around 50bc. the three festivals started to come together.
The result of the festivals coming together is our Halloween. When the Romans invaded France many of the Romans stayed there.
I'll point readers to my previous item on this (which links to other examples of this etc.) and remind folks that we do not know when Pomona's festival was (most likely in August) and that Feralia was actually in February. I'm all for finding Roman precedents in the postpostmodern world, but this ain't one of them.
UPDATE: Sad to see this misinformation uncritically examined in the Cal State Fullerton's Daily Titan, written in such a way that one might get the impression a prof from the religious studies department lended auctoritas to it ...
UPDATE II: And (arggh) is this misinformation what is lying behind the reference to Romans in the intro to an interview at Archaeology Magazine? (it isn't mentioned elsewhere in the interview)
UPDATE III: the Beacon Journal can't even get the purported Roman divinitrix right: "The harvest was celebrated each October with the Feast of Pompona, the Goddess of Orchards. "