Short answers, pulled from the story.
The evening before the 1st of November became known as All Hallows' Eve after Pope Boniface IV re-dedicated the Pantheon in Rome on the 13th of May during the 4th century. The Frankish Empire officially adopted the 1st of November as All Saints' Day in 835, which anchored the date for All Hallows' Eve.
Samhain marked the end of harvest season and beginning of winter for Celtic peoples in Ireland, Scotland, and Isle of Man. This festival began the evening before the 1st of November when the boundary between human world and Otherworld thinned allowing Aos Sí spirits to enter the realm.
Trick-or-treating evolved from souling and guising where groups collected soul cakes or food in exchange for prayers or songs. The earliest print use of term trick or treat appeared in 1917 from The Sault Daily Star in Ontario with widespread practice emerging in 1930s through US newspapers from Oregon and Montana.
Hollowed-out turnips have been carved since at least 16th century in Ireland, Scotland, and Northern England until immigrants discovered native pumpkins were softer and easier to carve. American tradition of carving pumpkins was recorded as early as 1837 in Nathaniel Hawthorne's story The Great Carbuncle.
Halloween accounts for over ten billion dollars annually in American economy with projections showing twelve point two billion dollars spent in 2023. National Retail Federation projects spending will rise to thirteen point one billion by 2025 while thirty million Americans spent four hundred eighty million dollars on pet costumes alone in 2018.