This is super fast. I just received the Interview Letter yesterday. However I have planned a trip to go back Hong Kong next week because my mother will undergo a surgery. My question is - can I call immigration to postpone my interview one month later because I was told that after filing the n400, we are not suposed to travel abroad. Or do I have to cancel my trip? Please advise..thanks everyone.

It's up to you if you to decide. If it's important for you to be back to HK, then you should go.
However, I can tell you that requesting them to reschedule your interview may delayed your process. You're very lucky to get such a quick respond to your application. You won't be able to decide when you can go in.. they actually will tell you when is the next available date is.

Good luck!!

You CAN travel internationally after filing the N-400. The only issue is that you must still meet the requirements for continuous residence and physical presence at the interview. A long trip could present a problem, especially if you were only just barely meeting the physical presence requirement. But for most people, a short trip abroad won't cause a failure to meet the requirements, so it's not a problem. At the interview, the officer will ask about any international trips you've taken since you filled out the N-400, so you should have your travel dates ready for him.

If you can arrange your trip so that you can return before the interview, it shouldn't be a problem at all.

If you need to reschedule the interview, that should also be possible. I'm not sure of the exact procedure, but I believe the interview letter contains instructions for rescheduling.

Yes you can travel and yes you can post-pone the interview. The thing is you don't get to determine when the re-schedualed interview is, they'll re-schedual that for you. People have gotten it weeks to over a year later or more. So that will be a risk you will have to take. Generally from what I've heard, is you get re-shuffeled back into the mix of INS and some people have gotten lost back in there.

So hope that helps...

Thanks for everyone's replied, I decided to reschedule the interview and go back to HK for about one month, hope my application won't lost when it re-shuffeled again.

Under urgent circumstances you can reschedule the interview ,do explain them your valid reason so that they know .Good luck!!