Imagine celebrating the New Year on Halloween. Ghosts, costumes, candy, parties, fortune tellers, bonfires- and champagne toasts at midnight! Our modern Halloween was not always about trick or treating and carving pumpkins. It was influenced by numerous other traditions, including the celebration of the Celtic New Year.
The largest influence on our modern Halloween is Samhain: the Celtic New Year celebration that fell roughly on October 31st – November 1st. Celts believed that on the evening of the New Year souls of the dead could return to Earth. Because the lines between the world of the living and the world of the dead were blurred, Celts also believed that Druid priests could make predictions for the New Year at this time and fortune telling became a common element of the festival. To commemorate the event, ensure good crops in the spring, and frighten away evil spirits, Druids built sacred bonfires where they offered crops and animals to the gods. They held feasts and invited friendly spirits to join them. Celts would dress in costumes of animal skin, most likely as a tactic to trick and confuse any evil spirits.  This is where we get our tradition of dressing in costume on Halloween. The food given to the spirits at the feasts could be one of several things that contribute to today’s trick-or-treating.
When Conquered by Rome, Do as the Romans
After the Romans conquered most of the Celtic territory, three Roman festivals began to mix with Samhain: Feralia, Pomona, and Lemuria. Feralia was the traditional Roman day to celebrate the Manes, or spirits of the dead. It was the last day of Parentalia, and therefore the last day the living had a chance to appease their deceased ancestors with offerings like grain, salt, corn, and flowers on their graves. It was also the only day of Parentalia that public feasts were held. It seems these offerings could also contribute to trick—or-treating today. Pomona was the Roman goddess of fruit trees, garden, and orchard; her symbol was the apple. It’s likely this contributed to the Halloween tradition of bobbing for apples. 
On Lemuria, spirits of questionable motives, possibly family members with unfinished business, came to the houses of Romans. There is debate as to whether these spirits were always evil or were unknown entities that could be good or bad. Either way, the head of the Roman household was tasked with making sure the spirits did not cause trouble and left as quickly as possible.  According to Ovid, worshipers carried out this ritual during the three-day holiday:
Rises (no fetters binding his two feet)
And makes the sign with thumb and closed fingers,
Lest an insubstantial shade meets him in the silence.
After cleansing his hands in spring water,
He turns and first taking some black beans,
Throws them with averted face: saying, while throwing:
‘With these beans I throw I redeem me and mine.’
He says this nine times without looking back: the shade
Is thought to gather the beans, and follow behind, unseen.
Again he touches water, and sounds the Temesan bronze,
And asks the spirit to leave his house.
When nine times he’s cried: ‘Ancestral spirit, depart,’
He looks back, and believes the sacred rite’s fulfilled. 
Having fulfilled the ritual, ghosts would take the beans and leave the house.
Asking strange spirits to leave your house and giving them food to persuade them? Sounds a bit like trick-or-treat to me! Though these holidays did not occur in the fall, it’s likely that the traditions still merged to influence how we celebrate Halloween today.
In the 8th c. C.E. Pope Gregory III declared November 1st the day to commemorate saints and martyrs, combining the days of the martyrs and the saints, which became All Saints Day.  He also moved the day from May 13th, the day associated with Lemuria, to its new date, possibly to align more closely with Celtic celebrations as Britain became more Christian. In 1000 C.E. the Christian Church made November 2nd All Souls Day, which was celebrated similarly to Samhain in the now-Christian lands of the Celts.  All Souls’ Day parades in England have also contributed to trick-or-treating. During the festivities, poor citizens would beg for food and families would give them “soul cakes” in return for their promise to pray for the family’s dead relatives. This was encouraged by the church as a way to replace the ancient practice of leaving food and wine for roaming spirits.7. All Saints Day came to be called All-hallows or All-hallowmas (from Middle English Alholowmesse) and the night before it – the traditional night of Samhain in the Celtic religion – was called All-hallows Eve. This, of course, became our name for the holiday: Halloween.
As we see, our most popular holidays derive from ancient and interwoven sources. Perhaps it is in our collective DNA as a species that through the ages we have heralded life’s rituals, shuddered at death’s emissaries, but still cast a hopeful eye to what lies ahead. So Happy New Year, Happy Halloween—or Happy Mash Up! Celebrate accordingly.