When I say Christmas can disappear, I'm not talking in the metaphorical sense, like your parents forgot to buy the Christmas presents. I mean that it is possible to experience a shorter version of it, or in the extreme case, no Christmas at all, due to travel around the Earth.
Let's look at an example. Flying from Los Angeles to Sydney takes about 15 hours. A flight leaves at 6pm Christmas Eve. In 10 hours it crosses the international date line. Just before the plane crossed the line, the time was 4am Christmas Day in Los Angeles (GMT -8), however for the passengers on the plane, the time was 12am (midnight) Christmas Day, since they were at GMT -12, just to the east of the international date line. When they crossed the line, time jumped ahead by 24 hours and it is now 12am December 26. Continuing to travel west, the plane will arrive in Sydney at 3am December 26 local time (GMT +10).
As you can see, the passengers on the plane have no experience for any time between midnight Christmas Day to midnight December 26.