June 1 is the day that a prospect’s future changes. The unsigned 2015 draft class CHL players’ rights expire on that day. The 2013 European players’ rights expire, as well, and the Toronto Maple Leafs have some unsigned players of each type to make a decision over.
Stephen Desrocher and Nikita Korostelev were both drafted from the OHL in 2015 — Desrocher from the Memorial Cup winning Oshawa Generals and Korostelev from the Sarnia Sting. Neither has been signed to an SPC.
Fabrice Herzog was picked from Switzerland in 2013; he hasn’t been around the team much, and has been pretty much forgotten by many Leafs fans, but he was still held by the Leafs.