What's more commonly used? "Dziś" or "dzisiaj"?
It's hard to see any difference. Something may look stylistically better in a specific sentence, or it may be because of personal preference, but they're really perfectly interchangeable.
Dzisiaj is usually more used in Polish
'It is sunday today'?
Does anyone knows Why this is not instrumental?
Because that would be as if you tried to define "today" by saying that it's a Sunday. But those don't mean the same, today just happens to be Sunday. Tomorrow it will be another day.
See here, part 5: https://forum.duolingo.com/comment/16373167
I wrote "the day is sunday". it would fit better if a question was being asked first. would this still be acceptable? Many thanks