All Hallows’ Eve, often contracted as Hallowe’en, is the eve of All Hallows (All Saints’ Day), and the first day of Hallowmas.  According to some scholars, the Christian Church absorbed some of the Celtic practices associated with Samhain and Christianised the celebration in order to ease the Celts’ conversion to Christianity; other scholars maintain that the Christian observance of All Hallows’ Eve arose completely independent of Samhain.

On All Hallows’ Eve, Christians traditionally believed that the veil between the material world and the afterlife thinned.

In order to prevent recognition by a soul, “people would don masks or costumes to disguise their identities”; in North America, this tradition is perpetuated through the practice of trick or treating. In Poland, believers are taught to pray out loud as they walk through the forests in order that the souls of the dead might find comfort; in Spain, Christian priests toll their church bells in order to allow their congregants to remember the dead on All Hallows’ Eve. The Christian Church traditionally observed Hallowe’en through a vigil “when worshipers would prepare themselves with prayers and fasting prior to the feast day itself.”  This church service is known as the Vigil of All Hallows or the Vigil of All Saints;  an initiative known as Night of Light seeks to further spread the Vigil of All Hallows throughout Christendom. After the service, “suitable festivities and entertainments” often follow, as well as a visit to the graveyard or cemetery, where flowers and candles are often placed in preparation for All Saints’ Day (All Hallows).

Source: Wikipedia