Against what I was taught and assumed correct, in fact there is no documented organic link between any European/British folkloric/pagan practices and the US children's holiday as it first appeared c.1900-1910. Actually, it is a distinctly American mass-cultural phenomenon: our 'Halloween' began in women's magazines and among literate school and church teachers organizing a new seasonal activity for young American children. The commercial marketing of this novel recreational diversion by candy-vendors was apparent almost from the outset, although "trick-or-treating activity" cannot be confirmed until the mid-1920s. (Unfortunately, recent Canadian claims of "First!" make some early dates very suspect, and unsubstantiated bullsh*t is mindlessly parroted in the media today. Why can't reporters check facts?!) The absence of Canadian evidence prior to US reports -- in fact, later, following by two decades -- strongly suggests a Cdn copying of its southern neighbor (again).
There were a few relevant essay articles in the late 1890s. One early reference I've found is from the literary, cosmopolitan (US) Methodist weekly newspaper The Epworth Herald (10/12/1901), describing folkloric (Scottish) traditions in the idea of hosting "Hallowe'en" parties for older American children; the unidentified author was probably a Scots-Irish but that is uncertain. Behind this article, rather obviously, James Frazer's The Golden Bough (1890) looms large. So this is a literate Victorian re-creation (mostly, wrong) and not a continuous, imported cultural practice witnessed live in any community here: it was just a party concept presented to adult (North American Christian) readers, for children.
Yet within two decades, an (American) Hallowe'en holiday would definitely be (re-)imagined through Frazer's greatly influential book. The oldest definite description of an actual American Hallowe'en event (i.e. a children's party, costumed ghoulies, w/ candy, jack-o-lanterns, a ghost story, etc.) is from the Green Bay WI church notes of a Christian Endeavor group event (held on Fri., Oct. 28, 1904) dated Dec. 1904. So already around the country, a few elementary school-teachers (probable readers of The Golden Bough) were exploring Hallowe'en possibilities c.1907. One typical example, in a NYC women's monthly magazine, The Delineator, a marketing ploy c.1909 known as "the Jenny Wren Sewing Club" explained a trite (un-)Hallowe'en party to (mothers of) young girls. It should be clear enough: in 1905, affluent/educated Yankee WASPs still didn't know what Hallowe'en was (or wasn't) simply because it had never existed here before! And so, this is commercial Halloween's true beginning: its novelty.
To my thinking, the first marketing of Hallowe'en candy (to adults!) is key: there's explanatory advertising around 1905 to the mid-1910s by merchandizers. I also see one report of a Hallowe'en "candy frolic" organized by adults for their boys in the (Panama) Canal Zone in October 1910, for example, but I'm still unclear exactly when kids' costumed 'trick-or-treating' began, all the b.s. of 'Firsts!' held aside.*
* Ethnic urchins' extortion or vandalism of merchants' property (1917) for "coin" isn't quite the same, sorry. (In Sault Ste. Marie, the population had doubled and children of these new immigrant Catholics were not from the UK or France; we cannot assume Eastern and Southern Europeans knew anything about rural Scottish folklore, either.)
A major impetus definitely, Radcliffe graduate Ruth Kelley was supposedly responsible for popularizing children 'trick-or-treating' nationwide in her epic The Book of Hallowe'en (1919). In fact, her book mentions nothing of the sort. She came from Lynn MA and there's also nothing to support the idea she saw local children collecting candy anywhere on the North Shore, Massachusetts around Oct. 31. On the other hand, she cites (in her Index of articles) sporadic accounts of Hallowe'en activities across the USA over the past decade. I think it's fair to say that our popular national holiday Halloween follows that, her compendious 1914-18 research, which informed early adopters of the children's holiday throughout the 1920s and '30s, almost a generation after the first reports. Hallowe'en was popularized in American public schools, by teachers apparently.
Many of our familiar Halloween tropes were recognizable by 1933:
In that same period, and into the confluence of trends, a spate of 1920s and '30s Hollywood horror-films solidified a spooky film genre in the public's consciousnesses, which I suppose led eventually to seasonal-commercial 'Haunted Houses' as a recreational entertainment thing nationwide, c.1958-63. The most famous single mass-event is probably still New York City's Greenwich Village Halloween Parade first held in 1973 (two generations after "Halloween" began).
As a wee-un in the 1970s, in Massachusetts, Halloween felt daunting and dreadful. Everyone had heard about the Salem Witch Trials, local history. Feminists/liberals effectively used that history (shortly before Native Americans began to 'correct' the proximate Thanksgiving celebration as a genocidal mourning day: another topic) as a cautionary note: Don't burn/hang people you disagree with. This, I recall: much of 'The Witches of Eastwick' scenery (filmed at my school and in my town in 1986) came from my own childhood in fact.
The imagery of the Witch Trials was scary, to a child -- step outside the norms, you were murdered by the Authorities. Growing up in the 1980s, that was in the background when Massachusetts formally abolished the Death Penalty (1982). Apparently, this work continues today.
In practice, Halloween c.1976-82 was when little kids of suburbia wore ready-made costumes and were terrorized by older teens who robbed you of your candy-boon and covered you in shaving cream or beat you up (fun times). I suppose that was common most places, from the 1940s -- the kitschy store-bought costumes from the 1970s on ...
In my opinion, the next level of commercialization was when SpookyWorld (with the real Elvira and Alice Cooper, in Berlin MA) arrived c.1991: it was a Big Deal, if not an absolute first, and fairly unique in the USA. Numbawan? Disney had theme-rides, this was different lol ... well, it was massive.
As an aside, to broach the literary and cultural sides of the horror connection, H.P. Lovecraft is correctly seen as a Providence RI author but his writing was set in Massachusetts locations, especially Essex County (including Haverhill and Salem MA). And from up near the New Hampshire border, (perhaps coincidentally) Rob Zombie of the NYC heavy metal band White Zombie was certainly not the first Horror Rock band, but by the early 2000s he had contributed significantly to the horror-gore genre and -- I am having a Mandela memory here -- his statue was erected in Haverhill MA c.2005??? I was aware of that as a North Shore thing, but he was always a rust-belt Massachusetts Kid Rock to me. Still, a Big Deal.
Which brings us to Salem, now known worldwide as "Witch City." (And USA Today declares: "#2 Tourist Trap in the World") When did that start? "Haunted Happenings," the organized event, is now 40 yo = began in 1983. I knew a few Wiccans c.1996 and I vaguely recall friends going up there to celebrate in the mid 1990s, around Halloween. But -- if this detailed history is correct -- even in 2000 the city itself was not promoting the Witchcraft stuff anywhere officially.
Everything changed after the Elizabeth Montgomery statue & controversy c.2005. Until then, Salemites reviled the "witch/satanic" stuff and it was still a very political tug-o-war. After that, KA-Ching!! There was waaaay too much money to be made (*now +35% of the town's revenue is earned in October) ... but the traffic/parking is terrible so Don't Even, honestly. In just two decades, that city has been transformed -- the Witches have won.
Of course, in late Sept. 2023 Salem is already ramping up for the Halloween hordes. And American trick-or-treating is now a thing in lots of other places too, even in Europe ... cue some shirty European to pitch in that Samhain was originally Celtic, blah, blah blah .. no, chief, that's NOT why you're carving a pumpkin and buying sweets for kids in Barbie or Spider-Man costumes this Oct. 31. Just sayin': this holiday is decidedly American, even if Dublin Ireland now hosts the world's best parade (first held c.1996?).
Get political, now? Salem's Satanic Temple is here, current location for the infamous Baphomet Monument.