Well, Halloween is a fall holiday where people dress up in costumes. Sometimes there are parties and sometimes you go to other people's houses and they give you candy. It's called trick-or-treating because you say 'trick or treat' when you knock on the door. [At least, this is what she's heard happens...very rarely did Nunnally leave Ashford.] You don't have to wear a scary costume, but Halloween decorations usually are.
And origami is a Japanese art. You can make almost anything by folding a square sheet of paper a certain way.