The First (and Only) Time Traveler Convention
By jsolof on May 07, 2005
Technically, you'd only need one, since if you missed it the first time, em... you could always come back. :)
Logically, the premise is hard to dispute. Announce the thing in media that will endure -- the organizers suggest acid-free paper notes stuffed into "obscure books in academic libraries" as one sure-fire approach -- and once they invent time-travel, having found the invitations, some future touristas are bound to come back and attend the first (and only) convention.
Impressively, it featured one of the world's most eminent physicists, Professor Alan Guth.
P.S. Thanks to Rocketboom for spreading the word! Now there's enduring media for you.