June 30 was chosen to provide a certain period of notice to owners and vendors of these firearms before the legal status of them would change. It provides time for people who may own the firearm to decide whether they wish to remain an owner of the firearm, whether they wish to comply with the requirements that would be in place to become a grandfathered owner. It also allows for vendors and importers to make their plans about what they may wish to do with inventories or imports of an item that is about to become a prohibitive firearm.
On May 8th, 2018. See this statement in context.