Today it is ... but what is Halloween?

Halloween (also spelled Hallowe'en) is an annual holiday celebrated on October 31. It has roots in the Celtic festival of Samhain and the Christian holy day of All Saints. It is largely a secular celebration but some have expressed strong feelings about perceived religious overtones.

The colours black and orange have become associated with the celebrations, perhaps because of the darkness of night and the colour of fire or of pumpkins, and maybe because of the vivid contrast this presents for merchandising. Another association is with the jack-o'-lantern. Halloween activities include trick-or-treating, wearing costumes and attending costume parties, ghost tours, bonfires, visiting haunted attractions, pranks, telling scary stories, and watching horror films.

Historian Nicholas Rogers, exploring the origins of Halloween, notes that while "some folklorists have detected its origins in the Roman feast of Pomona, the goddess of fruits and seeds, or in the festival of the dead called Parentalia, [it is] more typically linked to the Celtic festival of Samhain or Samuin (pronounced sow-an or sow-in)". The name is derived from Old Irish and means roughly "summer's end". A similar festival was held by the ancient Britons and is known as Calan Gaeaf (pronounced kalan-geyf).
Snap-Apple Night by Daniel Maclise showing a Halloween party in Blarney, Ireland, in 1832. The young children on the right bob for apples. A couple in the center play a variant, which involves retrieving an apple hanging from a string. The couples at left play divination games.

The festival of Samhain celebrates the end of the "lighter half" of the year and beginning of the "darker half", and is sometimes regarded as the "Celtic New Year".

The celebration has some elements of a festival of the dead. The ancient Celts believed that the border between this world and the Otherworld became thin on Samhain, allowing spirits (both harmless and harmful) to pass through. The family's ancestors were honoured and invited home whilst harmful spirits were warded off. It is believed that the need to ward off harmful spirits led to the wearing of costumes and masks. Their purpose was to disguise oneself as a harmful spirit and thus avoid harm. In Scotland the spirits were impersonated by young men dressed in white with masked, veiled or blackened faces. Samhain was also a time to take stock of food supplies and slaughter livestock for winter stores. Bonfires played a large part in the festivities. All other fires were doused and each home lit their hearth from the bonfire. The bones of slaughtered livestock were cast into its flames. Sometimes two bonfires would be built side-by-side, and people and their livestock would walk between them as a cleansing ritual.

Snap-Apple Night, painted by Irish artist Daniel Maclise in 1833. It was inspired by a Halloween party he attended in Blarney, Ireland, in 1832. The caption in the first exhibit catalogue:

There Peggy was dancing with Dan
While Maureen the lead was melting,
To prove how their fortunes ran
With the Cards ould Nancy dealt in;
There was Kate, and her sweet-heart Will,
In nuts their true-love burning,
And poor Norah, though smiling still
She'd missed the snap-apple turning.
On the Festival of Hallow Eve.

link: File:Maclise.snap.apple.night.jpg - Wikipedia, the free encyclopedia

Another common practise was divination, which often involved the use of food and drink.

The name 'Halloween' and many of its present-day traditions derive from the Old English era.

The term Halloween, originally spelled Hallowe’en, is shortened from All Hallows' Even – e'en is a shortening of even, which is a shortening of evening. This is ultimately derived from the Old English Eallra Hālgena ǣfen. It is now known as "Eve of" All Saints' Day, which is November 1st.

A time of pagan festivities, Popes Gregory III (731–741) and Gregory IV (827–844) tried to supplant it with the Christian holiday (All Saints' Day) by moving it from May 13 to November 1.

In the 800s, the Church measured the day as starting at sunset, in accordance with the Florentine calendar. Although All Saints' Day is now considered to occur one day after Halloween, the two holidays were once celebrated on the same day.