Hallowe'en (a contraction of All Hallows’ Evening) is thought to have roots in "Christian" beliefs and practices. However, where it reads "Christian," it should actually read Catholic. Beginning in the 4th century, the feast of All Hallows' in the western Catholic religion commemorated martyred saints. By the end of the 12th century, it involved superstitious traditions such as ringing church bells for souls in "purgatory."
All Hallows' Eve along with All Hallows' Day or All Saints Day (November 1st) and All Souls' Day (November 2nd), collectively called Allhallowtide or Allsaintstide, are a time when Catholics (not "Christians") honour dead saints and pray for recently departed souls who have yet to reach Heaven.
All of this, of course, is nothing but utter superstition and sheer nonsense. Most Catholic practices and beliefs are based on nonsense that the Bible condemns. There is no such thing as "purgatory." This life is all you get to repent of your sins and place your trust and faith in Jesus the risen Lord and Saviour. Once you die, that is it. You cannot pray the soul of a dead person into Heaven, nor can you purchase salvation for them from the "church." These are scams being conducted by the Catholic religions. This includes the scam where you pay a certain amount of money in order to "remit" your sins, while you go out and recommit those same sins the next day. That is not how forgiveness or salvation work!
People seriously need to learn how to think critically and investigate the practices of the various "Orthodox" Catholics, Roman Catholics, Lutherans, "Calvinists," Anglicans, Anabaptists, Presbyterians, Baptists, Episcopalians, Methodists, Pentecostals, etc. Many of them have beliefs and practices that do not square with the Scriptures.
Whether or not Hallowe'en was "Christianized" by the Catholic religion (they seem to like to "Christianize" everything pagan, yet merge the pagan with "Christianity"), everything about it is still based on paganism. Most beliefs and practices of the Catholic religion are rooted in paganism. Because the Reformers did not leave the Catholic nonsense in its entirety, or even other denominations like Baptists, many of their traditions are still rooted in paganism as well.