Halloween is not an official Holy Day in Medieval America, and is not exactly sanctioned, but it still retains some degree of popularity. As people become more superstitious, All Hallow's Eve is both darker and more necessary, and is usually a respite from the busy harvest season.
Houses are decked with the usual haystacks, pumpkins, and especially cornstalks, and the occasional scarecrows. Decorations are usually not much more elaborate, as resources can't be exhausted for me frivolities. They're usually decked out with agricultural byproducts.
Popular costumes include the usual ghost, devil, and witch outfit, as well as a clown or jester's suit. American monsters like Bigfoot and the Mothman are pretty common as well. Dressing up as a cowboy or pirate is done inversely proportional to local attacks one or the other wage on the local realm. Knight costumes are not roundly popular, as imitating or mocking a knights is a capital offense in many places, although it isn't unheard of for masters and servants to switch as means of fun/possibly deflecting the attention of evil spirits. For this reason, characters from American culture like Batman or the Jedi who have knightly aspects aren't usually dressed as. Spiderman is a more frequent, possibly because web patterned curtains and sheets are frequent this time of year, and easy to make a costume out of when they become worn.
Trick or Treating is only done in large towns, because it's safer there than big cities, but they have more resources than a village. Long gone are the days of candy bars, or even candy corns. Rather we have popcorn balls, pickled fruit, pynades, and for the particularly lucky, candied fruits. If a child is particularly lucky, he or she might get peanut brittle.
However, the celebration is geared towards adults, and wealthy ones at that. Costumes, plays, ghost stories. Decadent parties that have taken place of April Fool's Day and New Year's Even (which has become more sacred) as a time to go really crazy. At the end of the day, Halloween is a pastime of the upper classes, and the peasantry are very wary of Halloween. New Israelite almost never practice it, though they're not above using the full moon of October to tell a scary campfire tale.