Today, 31st October, is All Hallow's Eve (the day before All Saints Day) which is usually shortened to Halloween.
To Catholics, who do not believe that most of those who eventually get to Heaven can go straight there, having to go through a process called "Purgatory" first, Halloween was a vigil before the main feasts of All Saints' Day on 1st November, when the saints in heaven are commemorated, and All Souls' Day, usually on 2nd November (in some countries and traditions it is put back a day to Monday 3rd November when the second day in November is a Sunday) is a day for prayer for all the dead including those who are not yet in heaven and may be in Purgatory on their way there. In some countries the Catholic church also refers to All Souls' Day as the Day of the Dead.
N.B. One of the town and borough councillors in my division, Karl "Whammo" Walmsley from Mirehouse, has made the very good point that for road safety reasons, when children are going around doing "Trick or Treat" in the dark they should make sure their costumes are not all in black or dark colours only - wear something white or carry a glowstick or light. We absolutely do not want Halloween to lead to any extra funerals held sixty years early!
Here is Cumbria Police's advice for a safe Halloween