Well, I’m not sure they celebrate it the dame way in Hong Kong, but in the U.S. there are two basic varieties of Halloween.
The first is the Kid’s Halloween–children dress up and go door to door, saying “Trick or Treat” (in the old days, this was like truth or dare–if the adult said ‘trick’ the kid had to do something (like sing a song) before she got the candy, later, ‘trick’ turned into the relatively harmless pranks (TPing a house, turnign over emply trash cans) older kids would play on Halloween, and later still, 'trick came to be the not-so-harmless pranks (throwing eggs, turning over full trash cans, spray painting slow moving objects, etc)–but I digress). Now, “Trick or Treat” is just the code for “Give us free candy”. Kids sometimes have parties, and in a lot of places, they do their “trick or treating” at the local mall, sor safety’s sake. So, for an adult, unless you ae shepharding kids around, halloween is a night to open the door and hand out candy (you can deorate your house, or dress in a costume to open the door–try to out-scare the kids).
The second is the Adult halloween–usually one of the more raucous parties of the year. Often a costume party, Halloween has become a serious night for revelry. I’ve been to several that have been an annual event for years, each with it’s own little traditions. A “best costume” contest is almost de rigeour, with prizes for ‘best couple’, ‘scariest’, sexiest, etc. Sexy is a key word–for many women, ( a not a few men, I would guess) a great deal of thought goes into deciding whether they want to be scary or sexy. A few manage both–but I’ll leave taht to your imagination. If you are invited to a party, find out how serious they are about costumes!!! Wearing an elaborate get-up when everyone else is just in silly hats is not a good way to spend a party, and just showing up in a minimal outfit when everyone else has pulled out all the stops seems almost rude.
Really, local customs around the U.S. make it hard to say exactly what to expect at Halloween, but costumes and candy for kids, and costumes and alcohol for adults (and candy, for our inner child), are fairly standard.