I would make it before arriving at the restraurant just to make sure it's not a busy night. As always it depends on your party size too. We enjoy it here also.
I know when my parents went over Easter weekend they got a same day dinner ressie. They all really enjoyed it there.
My guess is that they probably picked up a cancellation.. so if you have any time at all I would at least try to get one ahead of time.
We had an ADR-free night during an Easter-time stay a couple of years ago, and Olivia's is what the concierge at the Poly found for us. It was quite crowded, but we really enjoyed it. I'd go for the ADR if you're going to be there at a busy time.