Akihabara massacre (June 8, 2008)
Refers to the specific mass stabbing incident that occurred in Akihabara, Tokyo, on June 8, 2008. Used as a proper noun for the event.
The Akihabara massacre occurred on June 8, 2008.
Compound of 秋葉原 (Akihabara, the place name) + 通り魔 (random attacker, street slasher) + 事件 (incident, case). The term was coined by media to refer to this specific event.