And another thing: Back in reality, the Feast of All Saints was already being celebrated in 373 A.D. to commemorate all the martyrs—those known and those "known but to God." At this time, most Celts were not Christian at all, but were painting themselves blue, and would continue to do so for another hundred years. All Hallows Eve is the vigil of All Hallows (i.e., All Saints). Big Catholic feasts were always celebrated with a vigil—such as Christmas Eve, the Easter Vigil, and Midsummer Night.
What? We ended that practice? No wonder people look at me strangely.
Yes, the Feast of All Saints predates the Christianization of the Celts, but it did not fall on 1 November. In the Christian East, the Feast of All Saints is a movable feast falling on the Sunday following Pentecost, and in the West it was originally a fixed feast falling in the Spring. The Patriarchate of Rome moved it to 1 November after the Christianization of the Celts (and before their schism) while the rest of the Church continued celebrating it in the spring.