The origins of modern-day Halloween can be traced back to the ancient Celtic festival of Samhain (pronounced sow-in) which marked the transition from summer to winter. The Celts, who lived 2000 years ago in what is now the country of Ireland, began their new year each November 1st but celebrated Samhain the night before the new year, when they believed that the lines between the worlds of the living and the dead became blurred. The Celts also believed that the presence of otherworldly spirits made it easier for the Druids, who were Celtic priests, to make all-important predictions about the future before the start of a long, cold winter.

To celebrate Samhain, the Celts lit massive sacred bonfires where they made sacrifices to the Celtic gods by burning crops and animals. People gathered together dressed up in costumes typically made from animal heads and skins, and they participated in fortune telling. At the end of the evening, they re-lit their hearth fires from the sacred bonfire to help protect them during the oncoming winter season.

After the Roman Empire conquered a large portion of the Celts' territory by 43 C.E., two Roman celebrations, Feralia and Pomona, were incorporated into the Samhain tradition. During Feralia, the Romans traditionally commemorated the passing of the dead, and Pomona was a day to honor Pomona, the Roman goddess of fruit and trees. The symbol of Pomona is the apple, which is believed to be the origin of the tradition of "bobbing" for apples practiced during Halloween today.

Around 735 C.E., Pope Gregory III expanded the Christian festival of All Martyrs' Day, put in place by Pope Boniface IV, to include all saints and moved the date to November 1st. Later, in 1000 C.E., the Christian Church created All Souls' Day, observed on November 2nd, to honor the dead--this is widely believed to be an effort by the church to replace the Celtic festival of the dead with a church-sanctioned celebration. Similar to Samhain, All Souls' Day involved bonfires, parades, and dressing up in costume. The celebration of All Saints' Day began to be also referred to as All-hallows or All-hallowmas (from the Middle English term Alholowmesse meaning All Saints’ Day). The night before it, the date of the Celts' traditional celebration of Samhain, similarly began to be referred to as All-hallows Eve and later Halloween.

In America during the 19th century, Halloween was much more commonly celebrated in Maryland and the southern colonies and not the colonies of New England (due to their strict Protestant beliefs). A distinctly American version of Halloween began to emerge after the beliefs and customs of the Europeans and American Indians blended together--celebrations included "play parties", public events in which groups of people shared stories of the dead, danced, and sang. Autumn festivities were common by the middle of the 19th century, but Halloween was not nationally celebrated yet at the time.

A flood of new Irish and English immigrants in the second half of the 19th century helped popularize the celebration of Halloween everywhere in the country, as Americans began the practice of trick-or-treating, practiced then by simply going to houses to ask for food or money, after taking from the traditions of the new European immigrants. At the turn of the century, America moved to make Halloween a more child-friendly and community-centered celebration, promoting Halloween parties that involved games, food, and festive costumes rather than focusing on ghosts and witchcraft. As a result, Halloween stopped most of its superstitious and religious traditions, evolving into the secular holiday that it is today.

Two of the most common traditions celebrated today during Halloween are trick-or-treating and dressing in costume, and both have European and/or Celtic origins. Trick-or-treating likely dates back to the celebration of All Souls' Day in England, when poor citizens would beg for food and families would give them "soul cakes" in exchange for their promise to pray for the dead relatives of that family. Children eventually took up this practice, called "going a-souling", and received food and money from houses in their neighborhood. The tradition of dressing in costume originated in Europe and has roots in Celtic tradition as well, because people believed that they would encounter ghosts if they left their house on Halloween, so they wore masks outside after dark to avoid being recognized.

Halloween has always been a holiday focused on mystery, magic, and superstition. The customs and superstitions of Halloween today are far scarier than the friendly spirits of the Samhain celebrations during the Celtic era. Crossing paths with a black cat is seen as unlucky, likely because of the belief from the Middle Ages that witches avoided detection by turning into cats. Walking under ladders, breaking mirrors, stepping on cracks in the road, and spilling salt are all also considered things to be avoided around Halloween.

In ancient times, many superstitions revolved around the future, specifically of young women who were hoping to find a husband. In Scotland, young women commonly tossed apple peels over their shoulders in hope that the peels would fall on the floor in the shape of the initials of their future husband, and they also stood in darkened rooms in front of mirrors while holding a candle in hope to see the reflection of their future husband's face in the mirror.

No matter what the time period, all of these Halloween superstitions rely on the good will of the same "spirits" who were worshiped by the Celts thousands of years ago.