Halloween is the vespers of the Feast of All Saints. That's literally what Halloween means, it is a contraction for Hallow's Evening. Hallow is an archaic English related to "holy", we still use it in a limited capacity, for example the traditional language of the Lord's Prayer has, "Our Father in Heaven, hallowed be Thy Name".
Since antiquity the Church has honored God's people, His saints, on days throughout the year. These days of remembering and celebrating the life of one of our heroes of faith traditionally fall on the day of their death, this isn't intended to be morbid, but rather they can be viewed as their "birthday in heaven", it is a way of honoring and celebrating their memory. These saints feast days have always been part of the Christian calendar.
But in addition to this, the Church often set aside a day to remember all the saints, all God's people who have come before us--whether we remember their name or not. Obviously not all of God's saints are famous, most aren't. Humble saints who lived their day-to-day lives in service to Christ. And so the Church set aside a day of the year, a day to honor and remember all of the saints.
The specific day this fell on, for much of Christian history, was not consistent. It often differed based on local jurisdiction, in some places it fell in April, in other places in May or June. In the Eastern Orthodox Church the day was eventually settled as the first Sunday after Pentecost, and is known as the Sunday or Synaxis of All Saints.
In the Western Church, the most common date for the Feast of All Saints was May 13th. This remained the most common date in the Western Church until the 700s, when Pope Gregory III had an oratory built in Rome which was dedicated to all of the saints, the dedication for the oratory was held on November 1st.
About a century later, another pope, Gregory IV, proclaimed the anniversary of the dedication the day upon which the Feast of All Saints would be celebrated, and so in the Western Church the Feast of All Saints was transferred to November 1st.
As is often the case of Christian feast days, it has been common that on the evening (the vespers) before the feast Christians gathered for worship and prayer. And so on the evening before All Saints Day Christians would gather together in church for prayer. And so the vespers of All Saints Day (or All Hallow's Day) became known as All Hallow's Eve, and then over time shortened to Halloween.
As is often the case, in predominantly Christian nations in addition to the specifically religious connotation of the feast days (and their vespers such as in this case), they became times for regular people to engage in some frivolity. Christmas, for example, was often a time when communities gathered together to play community sports (such as the origins of modern football/soccer), feasting, and drinking. And has often been the case throughout history, older children and teenagers annoying their parents and neighbors.
Which is why in the early 20th century in America, many people were tired of children pulling pranks every Halloween, and so shopkeeprs organized an event, children instead could dress up and go around asking for either a trick or a treat, to keep them entertained and let their parents deal with the sugar buzz. And it worked, turns out children liked the idea of free candy more than the idea of pulling pranks on shop keepers. And the practice grew from there.
Basically all of the modern connotations we have with Halloween are American. Which has often made many countries regard Halloween as a very "American" holiday, at least as pertains to the more cultural artifacts surrounding it--carving pumpkins, candy, watching horror movies, etc. All of these things largely became attached to Halloween through various reasons--as an autumn holiday the use of autumn imagery was natural, pumpkins, hay wagon rides, bobbing for apples. The nights are getting darker earlier, there's a chill in the air, pretty good time to watch some spooky movies.
So what about all the pagan and/or satanic stuff? Well, simply put, there isn't any.
Some have attempted to make a connection, and if you go on the internet you are going to get your fair share of websites making these claims. The one thing you'll notice, however, is that nobody is able to substantiate any claims.
Halloween has no pagan origins. Pope Gregory IV didn't choose November 1st for All Saints Day because ancient Irish Celtic pagans celebrated their harvest feast of Samhain on November 1st. We don't know what day the Irish pagans celebrated Samhain, and we aren't even entirely sure what they exactly believed about it. And by the time Gregory IV made November 1st All Saints Day in the 800's, Ireland had been thoroughly Christian for 400 years. So even that link can't be established. Not to mention other false claims, such as that the Irish pagans would dress up in costumes to scare away evil spirits. Dressing up on Halloween and going around asking for candy was, as already noted, something that arose only in the last century. It's not ancient.
What about the devil? So the founder of the church of Satan, Anton LeVey, basically got a kick out of making Christians really uncomfortable. If he were alive today, rather than in the 1960's, we'd call him a troll. He liked to be a troll. And one of the ways he trolled was to proclaim Halloween a high holy day of the church of Satan. It was a troll move, but someone intentionally being provocative and obnoxious. But it still worked, certain Fundamentalist Christians took the bait, ran with it, and began to associate Halloween as an evil satanic day. Why? Well Anton LeVey made it a high day in Satanism, so obviously it's evil now. Not sure why a troll claiming a day for the devil should be given any kind of weight or authority at all, but that's what happened.
It didn't help that, in the 1970's and into the 1980's there was an increase in various huckster charlatans claiming to be former practicing satanists, and inventing a a huge conspiracy theory about hidden devil worshipers in society. Leading to the Satanic Panic. Which is why every mom of the 1980's thought Dungeons and Dragons and children cartoons were secret gateways to satanism. And it's still why even today some Christians get their underwear all tied in a knot over things like Harry Potter and the Lord of the Rings.
