If I am addressing a card to multiple family members with the last name Thomas, is the correct spelling Thomas's or Thomas'?
The identical question was asked on this forum 12 years ago, right down to the name used:
Thomases. An apostrophe denotes ownership or a contraction.
Examples of correct uses:
I'm visiting the Thomases. That is Thomas' chair. That's the Thomases' dog. The construction "Thomas's" is wrong.
"Apostrophes are NEVER ever used to make plurals." (http://www.spotlight-online.de/CoCoCMS/generator/viewDocument.php?doc=4279) "Apostrophes are NOT used for possessive pronouns or for noun plurals." (http://owl.english.purdue.edu/handouts/grammar/g_apost.html)
from the MLA handbook, 5th ed.:
To form the possessive of a plural proper noun, add only an apostrophe.
[examples:] the Vanderbilts' estate the Dickenses' economic woes
last linky (http://webster.commnet.edu/grammar/plurals.htm)
When a family name (a proper noun) is pluralized, we almost always simply add an "s." So we go to visit the Smiths, the Kennedys, the Grays, etc.When a family name ends in s, x, ch, sh, or z, however, we form the plural by added -es, as in the Marches, the Joneses, the Maddoxes, the Bushes, the Rodriguezes. Do not form a family name plural by using an apostrophe; that device is reserved for creating possessive forms.