A gathering with the purpose of mass suicide, usually by members of a cult.
The phrase comes from the November 1978 mass suicide of the members of the Peoples Temple Agricultural Project, colloquially known as "Jonestown", in Guyana. Over 900 people died from ingesting Flavor Aid, a powdered drink mix similar to Kool-Aid, laced with cyanide and other chemicals. This is also the origin of the phrase "Drink the Kool-Aid".
"The Mayan calendar says the world is going to end. Let's have a Kool-Aid party!"