I didn't grow up here, so I have no clue. Do you just do sports all day? Like football, I have no idea.
In the US, our Sports days were called field days, and they were really fun. We had a waterslide, eight Popsicles, had a watermelon speed, sorry seed spitting contest so we ate the watermelons and save the seeds to spit them. Anyway, it was really fun. But it was more like a playtime, then a competition. I also remember we had a tug-of-war, and jumped rope to dance music.
So please, somebody tell me in the UK what sports day is like. And is it just for primary school? In the US, it was mainly for elementary students. I attended the residential special-needs school, so we did it until eighth grade I think. But usually you stop doing it at fifth grade. At least in Florida anyway. I don't know about different states.