"Somebody was playing the piano the day before yesterday."
Translation:Хтось грав на піаніно позавчора.
Not convinced that "позавчора" needs to be at the end. It does in English, but not in Ukrainian, I think
"Хтось грав на піаніно позавчора" sounds fine to me, but you can put позавчора elsewhere, of course, depending on what exactly you want to emphasize. The only position where it is clearly wrong is between на and піаніно.