The Halloween fascination with costumes answers a very natural human inclination.
I. Covering the Body
People, all people, since the heritage of Original Sin (and the loss of the original innocence which accompanies it in every man), attire themselves, i.e. wear costumes, of some manner or other, habitually, covering at the very least their "least honorable parts":
"The eyes of both of them were opened and they realized that they were naked. So they sewed fig-leaves together to make themselves loin-cloths." Genesis 3:7 (The Jerusalem Bible)
"Yahweh God made clothes out of skins for the man and his wife, and they put them on." Genesis 3:21
"It is precisely the parts of the body that seem to be the weakest which are the indispensable ones; and it is the least honourable parts of the body that we clothe with the greatest care. So our more improper parts get decorated in a way that our more proper parts do not need. God has arranged the body so that more dignity is given to the parts which are without it..." 1 Corinthians 12:22-25
Hence we are clothed from the moment we are born until the moment we die: every morning we get up (and wash) and dress ourselves. Among all the creatures in the material world man is the only one who has the natural need to cover himself (the slug and the hermit crab being perhaps exceptions, but even they do not decide to do this nor do they decide upon the nature and design of their costume which is readily provided for by nature). Man alone determines his costume. Only man dresses himself up, always and everywhere. He is embarrassed to go naked. And, if a certain man or society should not blush for his own exposure those who behold him will blush for him in shame, in pity or even in derision.
There is a curse by God attached to indecent exposure. It is "the curse of Canaan." It is the curse of sexual immorality. Nudists are never monogamous! There will never be perpetual virgins among nudists! It is impossible because those who refuse to dress like men and decide to expose themselves like beasts will also act like the animals in sanctioning and giving way to their unbridled passions, surrendering the good social order which is only based upon monogamous marriage.
In the garment culture of all of humanity we have an implicit confession of faith in man's superiority status in the material world and to the shame, exposure and vulnerability that comes from Original Sin, as attested to by the Bible.
Therefore, those who truly reject God are quite consistent when they wish to spend there lives in their natural shameful exposure. A true atheist must be a professed nudist, therefore, with all of the dire consequences!
So, Halloween, despite all of the dark and sinister trappings, is at it's very heart, a noble instinct which demands: attire thyself!
II. Covering the Face
The Halloween mask is about personality and anonymity. Here the motion is to pretend to be what one is not, or to pretend to disappear.
Usually it is in terror, but there are millions of American home-schooling families which are presently celebrating the Feast of All Saints on All Hallow's Eve and the children dress up to represent any one of the myriad canonized saints whose names and lives we know and wish to imitate, instead of the senseless and anonymous ghosts and goblins of the consumer culture.
The only real way to change one's "face" is to "put on Christ" (Romans 13:14): repent from sin and believe in the Gospel: thereby becoming a Saint! The real meaning of the mask is that the sinner can become a Saint, (and, unfortunately, vice-versa). Cf. Ezekiel 3:16-21.
The Vigil of All the Saints thus becomes an intelligent and bright Catholic Christian Feast again (in answer to the world's darkening thereof). And even the Satanists themselves, by covering themselves and dressing up give implicit testimony that God is God, and we are completely exposed and impersonal fictions without Him, like so many ghosts!
Clothe thyself and get a Face: be a Saint, and you become like God!