According to Alfred J. Kolatch in the Second Jewish Book of Why, in Judaism, Halloween is not permitted by Jewish Halakha because it violates Leviticus 18:3, which forbids Jews from partaking in gentile customs. Many Jews observe Yizkor, which is equivalent to the observance of Allhallowtide in Christianity, as prayers are said for both "martyrs and for one's own family". Nevertheless, many American Jews celebrate Halloween, disconnected from its Christian origins. Reform Rabbi Jeffrey Goldwasser has said that "There is no religious reason why contemporary Jews should not celebrate Halloween" while Orthodox Rabbi Michael Broyde has argued against Jews observing the holiday. Jews do have the Purim holiday, where the children dress up in costumes to celebrate.
The practice may have originated in a Celtic festival, held on 31 October–1 November, to mark the beginning of winter. It was called Samhain in Ireland, Scotland and the Isle of Man, and Calan Gaeaf in Wales, Cornwall and Brittany. The festival is believed to have pre-Christian roots. After the Christianization of Ireland in the 5th century, some of these customs may have been retained in the Christian observance of All Hallows' Eve in that region—which continued to be called Samhain/Calan Gaeaf—blending the traditions of their ancestors with Christian ones. It was seen as a liminal time, when the spirits or fairies (the Aos Sí), and the souls of the dead, could more easily come into our world. It was believed that the Aos Sí needed to be propitiated to ensure that the people and their livestock survived the winter.