Hallowe'en or All-Hallows-Eve takes place on the night of 31 October. Hallow is the old English word for saint.
On All Hallows Day (1 November) the Catholic Church remembers Christian martyrs. This feast day can be traced back to the Middle Ages.
All Souls Day - when Catholics pray for the 'souls of the departed' - follows on 2 November.
English Language level: Intermediate+ CEFR B2/C1
So is Halloween Christian or Pagan?
Like Christmas, Halloween combines Pagan & Christian customs.
The lighting of bonfires symbolized the plight of souls lost in purgatory (Catholic). It also scared away witches & ghouls (Pagan).
Souling was going door-to-door offering prayers for the dead. In exchange the visited house provides "soul cakes" and other treats. The modern form is 'trick or treat'.
Mumming (or "guising") consisted of parading in costume, chanting rhymes, and play-acting. The custom was originally associated with Christmas. Source
Halloween features in Victorian ghost stories like E. Nesbit's 'Man-Made-in-Marble' - read and/or listen to a retelling here.
But isn't the Catholic Church against Halloween?
Not exactly. It just does not recognise it as a religious holiday (again like Christmas Eve). Nor does it approve of modern attempts to connect Halloween with devil worship.
What about the modern festival?
Today the religious elements of Halloween eclipse the pagan ones. There are no Jack O Lanterns in the Bible!
Another example of the devil having all the best tunes ...