The date was chosen on purpose to rival Samhain and involved similar traditions, including dressing up as a saint, an angel or a devil and joining a parade, with the aim of weaning people off of the pagan festival. By the 9th century, Samhain traditions were still going strong, so the church tried to take-over the evening of the 31st of October itself by making it another feast day, "All Hallows Even" (evening) and this and Samhain (remember, it's pronounced something like "sow-een") merged to become "Halloween".

richmondparkchurch.org.uk an evangelical pentecostal church in Boscombe, Bournemouth, Dorset, UK.