Not trying to be a jerk here or anything, but when addressing holiday cards to a family, there is no need to put an apostrophe before the "s" when pluralizing the family's name. For instance: "The Smiths" is correct, "The Smith's" is not.
The apostrophe implies possession (or a contraction). Notice usage above with the phrase "family's name," Here the apostrophe is used because the name belongs to the family. Conversely, this post begins "Dear Readers," with no apostrophe needed.
Thank you all very much for the cards you've sent to me, grammar errors notwithstanding. It is very nice to be remembered.