Halloween originated from an Irish pagan festival known as Samhain

In pre-Christian Ireland, Samhain honoured the god of the dead, with sacrifices on October, 1' St, fearing that if they did not, they would die within a year. When Ireland became officially Christian, the Christian clergy tried to stop this feast, but, the superstitious Irish would not, so the Christian clergy changed the holiday around, by reinstating the Feast of All Saints to November 1' St (originally one week after Pentecost Sunday, which it still is, in Orthodox Christianity), and made All Saints' Eve, a day for appeasing malevolent spirits

Pagans are celebrating a pagan holiday. Christians come in and most of the people are converted to Christianity (by hook or by crook).

The people have a festival dedicated to Bob, the god of cookies, and they want to continue celebrating it because even though most people haven't believed in Bob, the god of cookies, for a long time (even before the Christians got there) they like having cookies and whisky and hanging out with their friends and family for a day or two. The local priest says, "OK we'll still do that but we'll call it the festival of St Macarius, the patron saint of desserts". A holiday is born!

The same thing happened with Halloween. Christianity spread to the pagans who celebrated the feast of Samhain for the end of the fall harvest. The Catholics came in and said "OK we'll still have a harvest festival but we'll call it All Hallows Eve and honor the dead saints rather than pagan gods". The people said "will we still be able to light a big bonfire?" The Catholics said "sure, party hard, but be all the earlier to confessional the day after!".

Halloween or Hallowe'en as it used to be written is All Hallows Eve. The evening before All Saint's Day 1/11. It is the night when it is believed spirits can visit the earth and may or may not cause m.k mischief and mayhem. I believe the reason for halloween costumes was to ward off the spirits. To confuse and confound them. I may be wrong about that part and anyone who knows for sure feel.free to correct me.