Plural
If referring to something about the Smith family as two or more, it would be the plural Smiths. Ex. The Smiths have two daughters, one son, a dog, a parrot, a hamster, and one snake.
*In the above sentence "The Smiths" takes the place in the sentence for "The parents".
In addressing, say a Christmas Card, it would be The Smiths.
Possessive
If referring to something owned by a person with the last name of Smith, it would be pluralized and then made possessive, reading Smiths'.
Example sentences of possession:
The Smiths' dog chased our cat up a tree.
The noise that kept me awake until 2 a.m. was the Smiths' dog.
When the Smiths held their New Year's Eve party, one of the Smiths' cars and one of the Jones' cars collided in the driveway. (bold is plural, the other two are possessive)
Johnny is one of the Smiths living on Second Street.
A roofer discovered that raccoons were living in the Smiths' attics and promptly told the Smiths about the various health and safety risks associated with removing the animals. (bold is plural, the other is possessive)
Assuming you're writing the card on behalf of more than one smith - use smiths
John and the Smiths took my mother on a picnic in the park.common nouns: mother, picnic, and park.proper nouns: John, and Smiths.
smiths
Morrissey was the lead singer of The Smiths.
The Smiths was created in 1982.
The Smiths ended in 1987.
The plural possessive form of "smith" is "smiths'".
The address of the Smiths Grove Branch is: 127 South Main Street, Smiths Grove, 42171 8130
A smith works with metal. There are gold smiths, silver smiths, black (iron is black metal), tin smiths, copper smiths, etc.
The first album released by The Smiths is eponymous, meaning that it is named after the band itself. So the first album released by The Smiths is titled "The Smiths".
The Smiths Is Dead was created in 1996.
Smiths Gore was created in 1847.