I bet this has been disscussed before but what I don't get is:
If adding an 's can be for something that is possesive as well as a shortening of 'is', then why isn't an 's added to 'its' when that's possesive?
e.g: Its head, not it's head?
And what else does this apply to except with 'it'?
I just confused myself

>And what else does this apply to except with 'it'?

all personal pronouns! (and this has been discussed before, ron added extraneously)

e.g., yours, his, hers, theirs, mine :)

I cast my vote with TEd here. His explanation is the one that makes sense to me logically; however, I must admit "one of the partners' birthdays" to me makes more sense aurally.

Aren't we all voting for the plural possessive? Am I missing something here?

edit Oh, now I see. We're arguing about whether birthday should be singular or plural. I gotta think on that....

1. One of the partners' birthday was celebrated.
2. One of his was celebrated.

The sense of the sentence is lost.

It's lost because you changed the meaning. The correct 2. should be

2. His birthday was celebrated.

His is replacing One of the partners' not the partners' birthday.

always...recast the sentence for clarity said TEd R, umpteen posts above in flat mode. Hear, hear.

Not to digress or anything, but somehow the original question brings to mind the futility of trying to understand the meaning of "the man who owned the lumbermill's daughter". (Probably it should have been the steel mill, because it sure needs re-casting.)

It's an ambiguously written sentence, and it sounds wrong either way, and we oughtn't be arguing about which wrong way is the correct one! (Now did I mean the birthday sentence or the one I just quoted in blue...)

The question here is whether One of is refering to one of the partners or one of the birthdays. One way out of the ambiguity is to recast the sentence as TEd suggested.

The birthday of one of the partners was celebrated.

One of the birthdays of the partners was celebrated.

DubDub:

Your sentence differs dramatically from mine, and both are correct. (though mine's more correct than yours )

"One of the partners' birthdays was celebrated."

Starts out as "One was celebrated." We don't know what it is yet that was celebrated, but when we add the main or primary modifier it becomes "One of the birthdays was celebrated." We know what was celebrated but we don't know whose. Then we add the secondary modifier and we have "One of the partners' birthdays was celebrated."

My sentence was "Today is one of the partners' birthday." The basic sentence is "Today is birthday." Yup. Not much information. We add the primary modifier "Today is one's birthday", grammatically equivalent to "Today is the birthday of one." We know now that we're dealing with a specific person, but no information about who it is. Then add the secondary modifier and we have "Today is one of the partners' birthday" or Today is the birthday of one of the partners." What's tricky here is that in English that pesky apostrophe s can't stay with "one" where it belongs. Other languages use better constructions to make clear whom we are speaking of. I cannot remember the details after all these years, but I'm virtually certain that the same sentence in classical Latin would be completely unambiguous.

I wish we had the ability to diagram sentences here. It'd be a lot clearer.

But both your sentence and mine are correct, though it's my firm belief that passive voice should not be used by us. If you recast the sentence as "We celebrated one of the partners' birthdays" it becomes grammatically equal to the sentence I used.

TEd

If anyone cares, the original sentence was Today's one of the partners' birthday.

How about the sentence, "I got a lift in one of the partners' car(s)." Can we differentiate between one (of the many) cars belonging to one (of the many) partners (cars), and one car belonging to one (of the many) partners (car)?

Bingley

i lose myself :(

Just quietly shut the door and tiptoe away, my dear. They'll stop raving sooner or later.

What if they both have the same birthday? Or, I know---suppose, like the Queen, they each have two birthdays on the same days as each other, one real and one official.