The purpose of a Blue Tuesday event is to refer to an actual event without disclosing any actual information of that event. As such a Blue Tuesday is an event that may or may not occur on a Tuesday and may or may not involve the color blue. Somewhat Similar to a surprise party, except people are limited to knowing it exists, and know nothing else about it.
Tim (to group): so about Blue Tuesday-
Sarah: whats blue tuesday?
Tim: you don't know about it, thats kinda the point.
Sarah: fuck you tim