Halloween, in the sense of the eve of all saints, is after Easter in the eastern churches, which is the 'old' date but was changed to coincide with the day that

Halloween, in the sense of the eve of all saints, is after Easter in the eastern churches, which is the 'old' date but was changed to coincide with the day that the poe consecrated the Pantheon as a Christian church (the Pantheon having used to be a temple to all gods, is now dedicated to all saints).