While it's not m y favorite chapter by far, I think Halloween and its buildup are one of my favorite bits of worldbuilding in Constellations, as it implies that not only is the mythology and cosmology that Sunny is familiar with real, but that the same rule applies to other regions and heritages.
Hence why she was not only taken off guard by the idea of All Hallow's Eve, but took it very seriously, considering she couldn't be certain what all might start crawling out of the woodwork in the United States, where there's a fuckton of possible lore to to have perpetuated itself among the various and sundry ethnic and geographical histories in the area.