The Celtic and Roman versions of Christianity agreed to unify at the Synod of Whitby in 664AD. Effectively Celtic Christianity got taken over on 1 November 664 - the feast of All Saints or All Hallows. The two different versions of Christianity marked the day differently. The Roman Church counted their days from midnight as we do today. The Celtic Church followed the Jewish practice of counting their days from sunset, so All Hallows Day was preceded the previous sunset by All Hallows Eve or All Hallows E'en. Some people in the Celtic Church decided that since their version of Christianity ended at sunset but they didn't become Roman Christians until midnight, they were freed from Christianity for a few hours and could indulge in pagan practices from days gone by. They had so much fun, they decided to do it every year.
The celebration of Hallowe'en in England was suppressed by the Commonwealth, but it continued in our American colonies who had a more tolerant form of Calvinism. Following the Restoration and the Gunpowder Plot, England instead burned effigies on bonfires on 5th November. In the late 20th century, Halloween (without the apostrophe) had become an American confection allowing aggressive begging by children dressed up as witches and it returned to England as an excuse to sell parents and partygoers stuff they didn't want.