JLMcAnally /Shutterstock So how did we arrive at trick-or-treating? Despite its early ties to the Celtic celebration Samhain (pronounced “sow-wen”), Halloween is a uniquely American holiday. Until recently it was only in the U.S. that we collectively mark October 31 by dressing in costume and sending our children door-to-door to ask the neighbors for candy, and even that is a relatively new tradition. On this week’s Gastropod , Nicola Twilley and Cynthia Graber team up with historians Joanne Freeman and Heather Cox Richardson, the cohosts of Now & Then , also on the Vox Media Podcast Network. Together, they look back at how Halloween became so big in the United States and, on a sweet note, how it became synonymous with fun-size chocolate bars. Freeman, a specialist in the Revolutionary War time period and its aftermath at Yale University, describes Halloween as “something to do with marriage, and capitalism, and candy, and Christian angst.” So yeah, about as American as it...