Come, pick my nit:
The twins marked their 25th birthday in Argentina on Saturday.
or
The twins marked their 25th birthdays in Argentina on Saturday.
There are two twins, and two birthdays (albeit the same calendar day). Neither option sounds right.
Unscientific answer:
Google search on twins “their birthday” = 47,100 hits.
Google search on twins “their birthdays” = 26,500 hits.
The grammar geek in me says that because they’re twins, they share a birthday (unless they were born on opposite sides of midnight, or the International Date Line, or a chronosynclastic infundibulum, or something), and thus “their birthday” is A-OK.
“Birthday”, singular, sounds preferable to me. Twins are generally considered to “share” a birthday.
Oddly enough, if it were two non-twin relatives whose birthday fell on the same day, I’d lean the other way: e.g., “My aunt and I, who were both born on 29 November, celebrated our birthdays together.”
Whatever the designated Usage Gnome may say, the twins are separate entities and are each entitled to celebrate a birthday.
Hence, “marked their 25th birthdays” sounds more righteous.
I am interested in what the Usage Gnome would say about the term “po-po’s”, which apart from the unnecessary apostrophe sounds like something my aged spaniel occasionally leaves a trail of in the kitchen.
Agree – but would suggest that it’s because the two births didn’t actually take place on the same day (i.e., not in the same year).
In both cases, the meaning is clear. Both usages are common; thus either is fine.
Here in the USA, and for English in general, there is no Authority. Thus, you are free to follow whatever common usage makes a clear meaning, generally choosing the one with the clearest meaning. True, at some work locations they will slavishly follow one of the “Manual of Style” books out there. In that case, whatever you boss sez to do is correct.
With twins, I’d go singular: there’s only one *day *being commemorated. With coincident birthdays of different years, I’d go plural.
As a twin, I use the singular. It’s the same day.
And it’s the “Grammar po-po”-- collective noun.
Agreed. It seems to be the word of the day around here, and I was just trying to jump on the Cool bandwagon. The apostrophe doesn’t make grammatical sense, but “Po-poes” is even worse.
My uncle and I have the same birthday, though he’s at least 45 years older than I am. So it’s Lil and G celebrated their birthday. For twins, it’s even more clear. they were born on the same day, they share the day. Exceptions would occur when the individuals celebrated the birthdays in different ways. But that’s just a usage thing.
-Lil
The plural of “po-po”, is “po-po.”
The subject, doesn’t need a comma, to protect itself, from other clauses. Gaudere always has the last laugh.
My style and usage hint: drop “po-po” from your vocabulary.
I’m not an expert, but I think both are OK. I would use a singular birthday if you are talking about the calendar day itself, but plural if you intend to refer to the celebration.
Also, “one-time” is stylistically preferred to po-po.
Who are you? The comma Po-po? 
I’m a twin, and we’ve always celebrated our birthday.
Sailboat