Which is correct:
- It's my daughter's, Mary's, birthday.
- It's my daughter, Mary's, birthday.
- It's my daughter, Mary's birthday.
- It's my daughter Mary's birthday.
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I think this is one of those questions that only an expert can answer. So I asked and @userr2684291 sent me to grammarbook. Thanks!
However, in common usage, we can share what we see . I don't see what a comma adds to any of these examples. Mary + daughter need no separation. They aren't listed. There are no separate thoughts that are being joined.
If today is Mary's birthday, we can say it a few ways.
It is Mary's birthday.
It is my daughter's birthday.
It is my daughter Mary's birthday.
Okay I went and asked. LINK
Rule: When an appositive is essential to the meaning of the noun it belongs to, don’t use commas. When the noun preceding the appositive provides sufficient identification on its own, use commas around the appositive.
Example: My friend Harvey is an animal lover. Explanation: My friend is not a precise identifier because one may have numerous friends.
Example: Harvey, my friend, loves animals. Explanation: Harvey is a precise identifier.
So: It is my daughter Mary's birthday. (You have more than one daughter.)
It is my daughter, Mary's, birthday. (You have only one daughter.)
The first one because the name is an optional information so you need to mark it off and your sentence has to make sense without it, too, hence you need the possessive 's with the word daughter.
It's my daughter's, Mary's, birthday.
It's my daughter's birthday.
Although personally I'd use em dashes here to avoid having commas so close to apostrophes.
It's my daughter's—Mary's—birthday.
It's my daughter's, Mary's, birthday. <- This one is wrong. It's either "my daughter's birthday" or My daughter Mary's birthday - never "My daughter's Mary's."
It's my daughter, Mary's, birthday. <- This might not be technically wrong, but it is very awkward.
It's my daughter, Mary's birthday. <- This is wrong. It's two unrelated phrases when you put the comma there "It's my daughter" and "Mary's birthday."
It's my daughter Mary's birthday. <- This is the most natural. This is how I would write it.
For what it's worth, I'm a native speaker and a Librarian.
It should be "It's my daughter's, Mary's, birthday."
Rule 6. If something or someone is sufficiently identified, the description that follows is considered nonessential and should be surrounded by commas.