Jack had received the invitation and was curious. Zatanna had mentioned the League's trouble with a vampire named Eric, so he couldn't help but wonder about this Eric who was having a wedding reception at Castle Dracula. Was it the same Eric?
He was curious, and he also felt that it was his duty as mayor to make an appearance and to extend the olive branch. If this man was a threat to the City, he needed to see. And if peace could be made to make sure that the people of the City were safe, he would do what was needed for that as well.
Jack had also wondered about the name of Eric's bride. He had met a Red once. Upon entering the castle, he approached the couple in question. He gave both a little bow and then offered his hand to Eric first. Now that he had confirmed that this was the same Red, he gave her a smile.