Halloween is "All Hallow's Eve", the night before "All Souls Day", the day Catholics pray for the dead.
Some Evangelical Christians have problems with Halloween, but I don't really see it. The idea that it is in any way connected to paganism (despite the claims of neo-pagans) is really silly. The modern practice was created by suburban (nominally Christian) moms who wanted fun and candy for their kids, and supported by candy companies. I can't prove that, but it makes a lot more sense than imagining neo-pagans in the 50's and 60's (during the height of tension with atheistic Communism).
I do have a problem with prayer for the dead:
"If any man see his brother sin a sin not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it." 1 John 5:16Some might say this is a description of mortal and venial sin. But the concept of different categories of sin is not well supported in Scripture (there are different punishments for sin, and the sin against the Holy Spirit).
The simpler explanation is that "death" simply means "death". If you see a brother sin, and live - pray for him. If your brother dies, don't pray for him.
Because there's no point: "And as it is appointed unto men once to die, but after this the judgment" (Hebrews 9:27). Your brother has gone to judgment, his case cannot be considered any longer.
Perhaps there is tradition in such prayers. Or they comfort us. I think such a thing can be done in a theologically sound way (assuming their life did not show fruit of repentance):
"Father, we pray for our dearly departed that, in their last moments, they might have come to a knowledge of sin, and trusted in You."
Or for a known Christian:
"Father, we thank you for the life of our dearly departed brother. We thank you for the testimony of their life, and for the reward they have now received."