Many of the ancient peoples of Europe marked the end of the harvest season and the beginning of winter by celebrating a holiday in late autumn. The most important of these holidays to influence later Halloween customs was Samhain, a holiday observed by the ancients Celts, a tribal people who inhabited most of Western and Central Europe in the first millennium BC. Amongst the Celts, Samhain marked the end of one year and the beginning of the next. It was one of four Celtic holidays linked to important transitions in the annual cycle of seasons.

Samhain began at sundown on October 31 and extended into the following day. According to the Celtic pagan religion, known as Druidism, the spirits of those who died in the preceding year roamed the earth on Samhain evening. The Celts sought to ward off these spirits with offerings of food and drink. The Celts also built bonfires at sacred hilltop sites and performed rituals, often involving human and animal sacrifices to honor Druid deities.

By the end of the 1st century AD, the Roman Empire had conquered most of the Celtic lands (see Rome, History of). In the process of incorporating the Celts into their empire, the Romans adapted and absorbed some Celtic traditions as part of their own pagan and Catholic religious observances. In Britain, Romans blended local Samhain customs with their own pagan harvest festivals honoring Pomona, goddess of fruit trees. Some scholars have suggested that the game of bobbing for apples derives from this Roman association of the holiday with fruit.

Pure Celtic influences lingered longer on the western fringes of Europe, especially in areas that were never brought firmly under Roman control, such as Ireland, Scotland and the Brittany region of northwestern France. In these areas, Samhain was abandoned only when the local people converted to Christianity during the Middle Ages, a period that lasted from the 5th to the 15th century. The Roman Catholic Church often incorporated modified versions of older religious traditions in order to win converts. For example, Pope Gregory IV sought to replace Samhain with All Saints' Day in 835. All Souls' Day, closer in spirit to Samhain and modern Halloween, was first instituted a a French monastery in 998 and quickly spread throughout Europe. Folk observances linked to these Christian holidays, including Halloween, thus preserved many of the ancient Celtic cutoms associated with Samhain.

Holiday traditions thought to be incompatible with Christianity often became linked with Christian folk beliefs about evil spirits. Although such superstitions varied a great deal from place to place, many of the supernatural beings now associated with Halloween became fixed in the popular imagination during the late Middle Ages and theRenaissance (14th to 17th cetury). In British folklore, small magical beings known as fairies became assocated with Halloween mischief. The jack-o'-lantern, orginally carved from a large turnip rather then a pumpkin, originated in medieval Scotand. Various methods of predicting the future, especially concerning matters of romance and marriage, were also prominent features of Halloween throughout the British Isles.

Between the 15th and 17th centuries, Europe was seized by a hysterical fear of witches, leading to the persecution of thousands of innocent women. Witches were thought to ride flying brooms and to assume the form of black cats. These images of witches soon joined other European superstitions as symbols of Halloween.