Roll the theme!
The word is sometimes spelled Hallowe'en, after a series of contractions. Let's work through them here. Hallow means sacred or holy, referring to the spirits of the dead. All Hallows Eve celebrates, you guessed it, all of the deceased ancestors and spirits. Eve is sometimes spelled even, which would have been contracted to e'en, with the apostrophe replacing the "v" sound. Take out the all and we get Hallowe'en. Boom.
Different countries and communities have widely-varying traditions around this changing-of-the-seasons time of year, but many of them have similar themes: Celebration and remembrance their ancestors, thanking the powers-that-be (God, gods, earth, spirits) for a bountiful and successful harvest, marking the changing of the season and the hours of daylight, and wishing for good fortune in the coming winter and following spring. In addition to the spiritual celebrations of Halloween night, we also have Diwali- the Indian festival of light, Dia de los Muertos- a Mexican celebration of deceased family, and a number of other festivals celebrated this time of year.
In ancient Ireland, the festival marked the coming of winter and its cold, crushing darkness. Ancestors were celebrated, but evil was also warded off with the carving of rutabagas into scary faces and the construction of huge bonfires to ward off darkness and to scare the evil spirits straight.
Archaeologists have found two probable sites for ancient Halloween celebrations: Tlachtga and the Hill of Tara. Both have structures pointing to the rising sun at the end of October and may have been used for celebrations of this autumn festival.
The vegetable carving survives today as the art of pumpkin carving, pumpkins being native to North America and thus not available to the ancient Gaels and later Celts of Ireland. The bonfire didn't make it over to mainstream North American Halloween celebrations, but young people in Ireland have kept this tradition alive, often building huge, unsafe fires in urban areas. Yikes.
Although technically illegal, bonfires in Ireland seem to be enforced just like drug dealing, illegal parking, and all other street crimes. They are discouraged, but not actively policed.
We might try some of these traditions this year, especially that of the fortune-telling bowl of colcannon. Simply, a trinket of some kind is mixed into a big bowl of colcannon (a mashed potato and cabbage mix) and the bowl is served to the party guests. Whoever gets the trinket gets good luck (or married, or will die, or any other superstition) for the next year. Cool!
However you celebrate the changing of the seasons, do so safely and responsibly! Happy Hallowe'en!