Question: What was held at the United Center in Chicago, Illinois on June 23-24, 2017 and resulted in Sebastian Aho playing under contract as a prospect of the New York Islanders?
Sentence (from "List of New York Islanders players"): Nearly 500 players have played for the team for at least one regular season or playoff game, and nearly 20 have had multiple stints over the years.
Relevant evidence: No