Those places are not nice, either for the people taking the animals in, nor the animals themselves. I believe they "smell death" there. The farmer on our fields had an 18 year old cow, he had her since a calf. She was past it, and booked in on the Sunday night. I made him promise to take her on the Monday morning, as I could not bear the thought of her spending her last night in such a place. He agreed he felt better doing that.
Its up to you, but if it were my pigs I would prefer to take them on the day itself, to avoid any stress both to the pigs and to me. I know some people will say thats silly, but if the pigs have to go, its nicer for them not to have to spend more time than necessary in that place.