The 1st of Nov is All Saints' Day, sometimes All Hallows' day in the Christian calendar where adherents would venerate and remember the dead who had risen into heaven. The celebrations would start at sundown the night before which was known as All Hallows' Eve. There is some dispute whether early christians in the UK adopted the Celtic Samhain festival which fell around the same time of year, but it was common, and easier, for a new religion to absorb the festivals and marked days into their own calendar to keep that continuity. So it is not unlikely.
Samhain was the time when the veil between the real world and the spirit world thinned, and ghosts, fairies and spirits could travel to this world. It also marked the end of the harvest and a reminder of the coming of winter. In Scotland especially, this mean long nights of darkness for several months. For a superstitious people, who had to rely on their harvest to see them through the next 6 months or so it was a time to offer some of the harvest to the supernatural entities that had governed their survival for centuries. From the 16th C onwards several traditions cemented in Scotland and Ireland that some still follow today. Guising ( 'Disguising' - dressing up and visiting neighbours to demand food or drink) often carrying a hollowed out and grotesquely carved turnip, playing pranks and scaring people, along with celebrating and playing games involving apples or nuts. Robert Burns' poem 'Halloween', written in 1785, tells of bogies (ghosts) and detailing 'fearfu' pranks' all happening on 'that night'.
The practice was taken to the USA by Scots and Irish immigrants and it was the Americans who turned it into the festival we see today, with 'Trick or treating', carved pumpkins (have you ever actually tried carving a turnip!) and costumes invoking modern horror tropes of ghosts, werewolves, vampires etc. The American version spread through movies and TV shows around the world in the past few decades and has now largely replaced the traditional Scottish pursuits. But, remember, if you came across a ghostly figure tonight on your travels, it may not necessarily be someone dressed up, it may well be a real ghost. After all, what better night than tonight to roam the world hiding in plain sight. Happy Halloween everyone and remember to brush your teeth before you go to bed.