Halloween is based on traditions from the Celtic festival of Samhein where people would light bonfires and dress in costumes to ward off wandering dangerous spirits as well as being a time of harvest. If you're interested in the connections of this ancient pagan festival to today's events, you might want to find The Book Of Hallowe'en by Ruth E. Kelley (1919), Halloween: An American Holiday, an American History by Lesley Bannatyne (1990) or Traditional Festivals: A Multicultural Encyclopedia, Volume 1 by Christian Roy (2009).
The concept of Halloween as we would recognize it came about in the early 1910s. Thanks to the hilarious series of "Drunk History", you might have already been aware of what the story is behind "trick-or-treating" in the sense that we know. Basically, one woman had enough of her garden getting pranksters' wrath and so decided to sponsor a party so awesomely sugar-infused and packed with alluring games and activities that essentially she distracted them all night from going on a version of "The Purge" on her heirloom roses. And parents liked that it kept their kids contained and happy, the town was cool with the idea of not having as much vandalism (and possibly generating some revenue from events), and her plants survived the night, so it just...continued and spread across the country.
Of course, "Drunk History" explains it in a far more amusing manner, so click on this link to check out their Halloween episode for more laughs.
By 1935, Halloween was still known as "Hallowe'en" and people were making homemade costumes and "trick-r'-treating" had been around long enough that it was forming into the current tradition - the first known print reference to "trick-or-treat" was in 1927. This was in the Alberta County Herald so not technically from the U.S.A but is North American... At least it's liked better in Canada than in the UK as apparently, while Halloween is a thing, it's hated over there! Apparently, it can be viewed as "too American" and steals the spotlight from the U.K.'s Bonfire Night. Oh well, more candy for us.
Full-scale creepy-themed decorations had been a prominent part of the 1920s when the Beistle Company produced a Halloween-specific line of party decorations. This continued into the 30s with Bogie Books, published by the Dennison Manufacturing Company that offered craft and party ideas solely for Halloween. The 30s are also the era of the Golden Age of Monster Movies too, with Dracula (1931), Frankenstein (1931), King Kong (1933) and The Invisible Man (1933). But, even so, one of the most popular costumes in the 30s based on a fictional character turned out to be Mickey Mouse (debuted in 1928). While trick-or-treating waned during World War II due to sugar rationing among other reasons, it was promptly picked up and has carried on steadily to today.
In the Waterford Historical Museum and Cultural Center's collection of documents is this "Hallowe'en Greetings" from October 30, 1935 of the Waterford High School newsletter.
Inside are the usual columns, with just one section dedicated to mentioning Halloween as "Hallowe'en". But it is interesting to read the "Movie Reviews" , "Fashion Flashes", and other things they felt worth noting, although the "Did You Know That?" and "Home Rumors" sections read a little like a Gossip Girl narration with lines like:
"Say, did you see Hank Button with Ruth Rebman? My "G" Men on Middle Street tell me that this has been going hot and heavy for quite some time."
"Oh! was I surprised when I saw Vera Currier, one of the few silent women, dancing around with her brother! She danced like Ginger Rogers and he wasn't so hot either."