I'm not that old, but when I was a child/teen, stadia was the common term. As in:
- Wembley, the Nou Camp, and the Santiago Bernabeu are football stadia.
- The MCG and Lord's are cricket stadia.
But now stadiums is the more common term. Is this shift essentially redundant? I've read some sources say that it better reflects English language pluralisation, but then I think "so what?" Stadia better reflects the Latin root, and English isn't a wholly logical/systematic language (very few languages are). If we call more than one dog dogs, more than one ox oxen, and use cattle both as singular and plural, then why does it matter?