Just to clear this up
Most (objective) historians believe that Hallowe’en originated as the Celtic “Samhain” celebration of the end of the light half of the year, and the beginning of the dark half. The border between the two phases of the year was believed to allow spirits into our world from the “other” world. The wearing of costumes and masks was intended to ward off the evil spirits.
As with many other festivals (such as Easter, which was also a Pagan festival), the major religions have organised their own celebrations on the same dateon purpose - with the aim of taking attention away from them, marginalising them, and eventually subsuming them.