"You guys have never celebrated Halloween!?" Angelo asks his Nephilim friends in a panicked shock. They shake their heads which leaves Angelo even more shocked because his mouth keeps going wider and wider.
The thought of them never celebrating Halloween doesn't sit right with him because it's one of the biggest holidays in America. Sure it's good that they celebrate Christmas and Easter but Halloween is cool and Angelo thinks it should be celebrated. At first, he thought that they should be celebrating Halloween since they know so much stuff about demons but then he remembers that they hate demons. It's in their blood contract to kill them.
Maybe that's why, but Angelo doesn't want to jump to conclusions.