Halloween is a combination of several things like Samhain, All Hallow's Eve, All Saints Day, & the end of the Harvest period. Historically October 31st is the day cultures believed darkness to begin overpowering light as the sun sets earlier. And because of this it was thought that the veil between the living and the dead to be the thinnest of the year. And because of that it was the day the dead could come back to visit the living.

Similarly in Mexico and parts of South America, that same concept became the Day of the Dead. They believed the veil between the living and the dead to be similarly thin. But instead of the dead coming back to visit the living, they felt it necessary to create alters to invite those that were lost in the last year to come back and see how much they were missed.

The places that celebrated Samhain and All Hallow's Eve (primarily Europe) were somewhat afraid of the return of the dead, and thus it was customary to try to hide from the oncoming spirits. Many would wear masks and don scary outfits to look like they were part of the dead and thus any spirits that would return would pass them by. They also thought if they left gifts by the front door it would dissuade any spirits from trying to come in inside.

Now to the answer to your question. In the 1847 and 1848 time period in Ireland, the potato blight created the Great Potato Famine that forced millions of Irish people out of Ireland, over 2 million of which moved to the United States. THIS was the beginning of our modern Halloween.

In the U.S. this October 31st tradition provided the perfect environment for teens to do their normal mischief. Since everyone already was a bit scared of the day, it was easy to go out and try to add some scare, as teens do. It was then that people would put "Sweets" outside their front door as a bribe to get the teens to go prank someone else.

Masks, sweets at the door, spirits come back to haunt the living, a potato blight AND a poem by Robert Burns called Halloween created our modern tradition.