They're all centered around the Griswold family, with Clark Griswold (Chase) as the father. The ones I've seen...didn't realize there were 7!...are all stand-alones. If you have ever seen Chevy Chase, you already know all you need to know about the backstory.I've never seen it. I've just looked it up and apparently the "national lampoon vacation" movies are a bit of a franchise. There are seven in total from 1983 to 2015. Your Christmas one is third in the list dating back to 1989.
Are they all interlinked, meaning do you need to watch them all in order to get the gist of them, or are they all stand alone unrelated films?
You are in for a treat!