Last year I noted Nike’s use of “sport” (rather than the traditional American “sports”) in a social media campaign. Last night was the first time I’ve seen it on TV, in a Nike commercial in ESPN’s coverage of the U.S. Open tennis tournament.
“Sport” Proceeds Apace
notoneoffbritishisms.com 
It has just struck me that, here in England in the 1970s, we had an annual “sports day” at school; all the pupils competing at various sports throughout the day. We would be wearing sports kit, carried in various sports bags. Was that peculiar to the Birmingham area?
Same for me in the north-east in the sixties, I think. (It was a long time ago.)
But that was the use of the plural – more than one sport was involved.
I think the use here is more like in the phrase, “I like watching sport on television.”
Nope, we had a sports day, too, in London. Somehow BrE differentiates between “sports” — lots of different sports — and “sport” — the field of endeavour. The distinction is clearly wafer thin, but somehow there is a rule we unconsciously apply.
‘Sports day’ is standard in Britain. Perhaps it’s because we want to convey ‘a day of our sports’, the sports that we play in our school or village’ and not the more general notion of ‘sport’, encompassing all and any sports.
Newspapers will have a main heading ‘SPORT’, along with ‘NEWS’, ‘FINANCE’ etc but under that ‘Sports News’. We would
say ‘the sports section’ or ‘the sports pages’.
Indeed, I have the Sport section of The Guardian in front of me right now.
I just perused Oxford English Dictionary entry for “sport,” which also includes “sports.” It was kind of dizzying. As the commenters have indicated, in Britain, “sport” and “sports” are both used, depending on the context, including for “sports day,” which has this definition and citations:
sports day n. a day of organized sports for a particular group of people, esp. an occasion on which the pupils of a school compete in various races and athletic events.
1875 Ipswich Jrnl. 12 June 5/5 Both Friendly Societies..[decided] to hold a counter demonstration in the town on the Sports Day.
1906 Times 17 Jan. 11/6 It may interest you to know that about two years ago my company instituted a competition amongst passengers on Sports Day on board its steamers for the quickest and proper method of putting on the lifebelt.
1924 Times 22 Mar. 6/3 Shrewsbury School Sports Day will be on Saturday, March 29.
1940 F. Sargeson Man & Wife (1944) 54 When the last war ended I was at the High School. We got the news of the armistice on our annual sports day.
1990 Independent 26 May (Mag.) 7/3 I remember sneaking off with a friend during Sport’s Day at my school.
(I think the apostrophe in the last quote is a mistake on The Independent’s part.)
In the U.S., by contrast, it is always “sports.” “I like sports,” “sports is [or is it are?] important,” “sportswriter,” “sports coat,” “the sports page”–and the heading on the last is “Sports,” not “Sport.” I can think of only two ways “sport” is used here. The first is referring to a particular endeavor–“baseball is my favorite sport.” The second is to refer to an individual: “he’s a good sport,” Gatsby’s “old sport.”
I think the jacket is always called a sports coat here in England, yet the Marty Robbins song is called “A white sport coat”. Was that something different over there ? Or a very early OOB.
I have never (Eng) heard “sports coat”, always jacket. In the US it is “sport coat”, I believe?
All four forms used in U.S.; the most popular (interestingly) are sport coat and sports jacket. https://books.google.com/ngrams/graph?content=sports+coat%2Csport+coat&year_start=1800&year_end=2019&corpus=28&smoothing=3
Is it not “sports’ day” or possibly “sport’s day” ?
If it had started out as ‘a day of sports’, that would translate grammatically into ‘sports’ day’. That led me to wonder about whether it had become ‘sports day’ simply because it’s slightly easier to say than sport day. Another possibility is that the apostrophe was dropped somewhere along the way. I don’t think it’s used, is it?
I get the impression that until sometime in the last century, you tended to use the ‘s possessive form only for animate beings: the man’s foot, the dog’s bollocks, etc. I remember the Austrian Liverpudlian musician Fritz Spiegl complaining on the radio in the seventies about a newspaper report referring to “London’s Royal Festival Hall”. People say a book cover, not a book’s cover, for example.
English East Midlands: Yup, it was always ‘sports day’ at my schools; easier and clearer to say than ‘sport day’ I should think.
Sports headlines you wouldn’t see today – Out of my way, wench! – Fur for the Cox – CULNT OK – https://www.varsity.co.uk/sport/12760
How knowing these were, I’ll let you judge for yourselves.
I’ve just read this post and all the comments, and the words ‘sports’ doesn’t look like a real word anymore.
Never seen sports coat in UK, only sports jacket. Doesn’t US English use coat the way UK uses jacket?
Bespoke tailors in the UK call the item a coat, both as an individual garment and as a part of a suit. Everyone else calls it a jacket. Unfortunately, proper tailored menswear is now a rare sight on the streets of both UK and USA.
I have an old fashion guide for men which states (from memory): “Monkeys and waiters wear jackets. You wear a coat.”.