Gemini wrote
That depends on whether you are talking about a group of people or talking about something owned by them. Part of the problem is that the plural and the possessive sound almost the same, and in some usages they sound exactly the same.
As I was taught in school:
The plural of "Hollands" is "the Hollandses." The possessive of Hollands is Hollands', as in "The Hollandses [all of the people] all share equally in the use of the Hollands' vacation home [possessed by all of them].
Compare the common Welsh last name, "Williams". The family group are the Williamses. The house they own is "the Williams' house".
In an older form, the possessive was "the Williams's house", which sounds _exactly_ like the plural. I still think that's better than "the Williams' house", which sounds as though the house belongs to the head of a clan, called "The Williams" the way the head of Clan MacGregor is called The MacGregor (for instance). However, I'm just the one who got good marks on the exams; nobody listens to me :-)
It gets worse when people put up signs labeling their home. If the name were "Holland", their name as a group would be "The Hollands", and the house name could be "The Hollands' [house]". When the name is "Hollands", then the name as a group would be "The Hollandses" and the house would be "The Hollandses' [house]". In reality. the signs that I've seen just say "The Hollands". (That implies that the lawn with the sign on it _is_ the family.) It's just as well; doing it right would just cause trouble because people who don't know the rules would assume the name was spelled "Hollandse" and the confusion would be endless.
=Tamar, in a picky mood