Vector Day was a yearly celebration. It was common knowledge that Mrp-Mrp Poo would host festivities for this holiday inside the Boulderdash terrarium.
The book Monsters and Aliens from George Lucas by Bob Carrau, released in 1993, made reference to Vector Day. Leland Chee, an employee of Lucasfilm who is responsible for the Holocron continuity database, has stated that the information within the book is considered non-canonical.