I was doing exercises from Murphy's textbook and came across the following sentence:
It was Ken's birthday yesterday. We had a party.
I should have changed 'it' to 'there' where it was necessary, so I replaced it to 'There was Ken's birthday.." because the grammar says that
We use there ... when we talk about something for the first time, to say that it exists
But the right answer was that this sentence was OK with 'It' at the beginning. I was surprised, because there hasn't been any mention about Ken's birthday before and they say about it for the first time. So why can't we put 'There' instead?