I understand we should use has/have been for something which started in past and still continuing in present. But is it right to use for something which started in past but has just completed or completed a few hours ago? For example - I want to tell someone in what I did the whole day. Should I say - I have been in meetings all day or I was in meetings all day? May be I should use "was"? Because day is already over? Please suggest. Thanks.
You can use have been when something has just recently ended. For instance, you would say:
I'm exhausted because I've been in meetings all day.
was would also be acceptable, but I don't think it's as common in this case.
But if it was in the more distant past, you must use was:
I didn't get any writing done yesterday because I was stuck in meetings all day.
It would be wrong to use have been here.