This is a question that has multiple answers.
As an example, if you are in the military, you may well live in one diocese, but you are ALWAYS under the jurisdiction of the Military Ordinate. The rules set up by the Military Ordinate will apply to you, no matter where in the world you happen to be physically located.
I ran into this while stati0ned in Germany in the early 1960’s. I did a significant amount of work at a local convent, and one day my buddy and I brought sandwiches out on a Friday afternoon. Mother Superior was scandalized that we were eating meat on Friday. I pointed out to her that we fell under the Military Ordinary (then the Archbishop of New York), who had waived the rule about not eating meat on Fridays for all members of the US Armed Forces.
She took this to the Bishop of Bamberg, who informed her that yes, all soldiers of the US came under the rules of the Military Ordinate.
For civilians, you come under the jurisdiction of the Bishop that covers where you reside. The same rules would apply as if you were traveling on vacation. If, in your diocese, New Years Day is a Holy Day of Obligation (not waived), then it would apply to you.
Of course, Sunday is ALSO a Holy Day of Obligation in a sense, as you are required to attend Mass on Sunday’s too.