Halloween is un-Catholic, which is why so many traditional Catholic priests have taught over the years, that Catholics should not observe this holiday.

Halloween, which the pagans call Samhein, is an ancient pagan feast still celebrated on October 31st, by witches and other pagans, and which pagans usually describe as being the most important feast of their (false) religions. See, e.g., “I’m Wiccan and This is What Halloween Means to Me” & witchway.net

Besides the opposition “on its face”, between Catholicism and paganism, there are many other ways that Halloween is the opposite of Catholic:

• The vigil of All Saints Day is a day of penance: a traditional fast day. Halloween is the opposite: it is portrayed as a day of gorging, of candy and of eating in between meals (mortally sinful on a fast day).
• The Catholic Church takes witches, spells and demons very seriously and requires Her children to stay far away from them. Halloween is the opposite: it makes witches, spells and demons seem approachable, fun, familiar and harmless.
• The days which Catholics celebrate are about life and salvation. For example, Christmas is about our Lord’s birth in His humanity. Even Good Friday is about our Lord’s life-giving sacrifice on the Cross, out of love for us, to open the gates of heaven. (This life-giving sacrifice of love, which makes salvation possible, is why the Church calls this day Good Friday.) In complete contrast to Catholic celebrations, Halloween is largely about death and destruction.
• Catholicism takes death very seriously. The Church urges us to solemnly meditate on death and prepare for it. Many saints kept a skull in their bedrooms, to ever remind them they were on earth to prepare for death. Halloween is the opposite: it is a time of smiling skeletons, tombstones with funny epitaphs, and a light-hearted treatment of death without any of its eternal consequences.
• Catholicism takes sin very seriously. The Church admonishes us to have a horror for sin and to consider it as the only true evil and unmitigated disaster. Halloween is the opposite: it is a light-hearted treatment of sin, e.g., with costumed attackers randomly committing unprovoked mayhem, with lots of blood and gore, all without the consequences of reality.
• Catholicism values beauty and order. Halloween is the opposite. Halloween glorifies ugliness and disorder, e.g., grotesque, painted-on scars “decorating” ugly and horrifying monsters.
• Catholicism values peace. Halloween is the opposite, exalting sudden and unprovoked violence, all without the consequences of reality.
• The Catholic Church forbids séances and all attempts to conjure the dead. Witches and other pagans believe that the feast of Samhein is when the boundary between the worlds of the living and dead is blurred, and when the ghosts of the dead can return to earth. Id. This pagan belief is honored by Halloween’s ubiquitous ghost decorations and costumes.
• The customary greeting of children seeking candy, is “trick or treat”. Think about this. However unthinkingly this phrase is uttered, it is in the form of a threat, viz., if you don’t give me candy, I will do something you won’t like!
• Spiders are among the most common Halloween decorations. Spiders are prominent pagan symbols and are considered as guides in the occult. See, e.g., druidry.org & whats-your-sign.com

All of the above considerations leave entirely aside Halloween’s worldliness, consumerism, immodest costumes, etc.

Because Halloween is in many ways the opposite of Catholic, it is no surprise that Halloween is ever-more popular, as society sinks ever-further from true Catholicism. See “Halloween is big business” & “Ghoulishly good news for the Halloween economy”

Someone could reply that Halloween is “all just in fun” and is not meant to be serious. We reply: if a Catholic is willing to participate in un-Catholic things which are “all in fun”, where will he draw the line? If the practice of getting candy involved stamping on a crucifix “in fun”, would that be OK? How can we ever re-conquer society for Christ the King, if we take part in anti-Catholicism “in fun”?

So, what should a Catholic do? Do not take part in Halloween! Instead, celebrate All Saints Day even more than before! If there are “trick or treaters” where you live, we suggest you hang a sign on your door on Halloween, which says:

Dear Neighborhood Children:

Our family is Roman Catholic and so does not observe the pagan festival of Halloween. Therefore, we do not give out candy today.

However, tomorrow (November 1st) is the great Feast of All Saints and we would be very glad to see you then and give you candy, if you wish to come. Please come between 1pm and 8pm.

No costume is necessary. However, if you decide to dress up as a saint, we will gladly be even more generous with candy, to reward your efforts.

Wishing you all the best!

Your neighbors,

[your name here]