Well "Happy Twilight Day" was posted on (http://www.xsparkage.com/?p=329) on November 20th, 2008 at 3:35 pm, so I'd have to guess November 20th. I really don't know anything about this though because I cannot stand Twilight, or Twilight fans.
Also (http://patterson-a-team.blogspot.com/2008/11/happy-twilight-day.html) posted on November 21st, 2008, so it's somet ime around there.
P.S. twilightsagafan: Clearly there is a "Twilight Day," granted it's not a FEDERAL holiday, and if you really were "THE BIGGEST TWILIGHT SAGA FAN EVER" wouldn't you have known there was a Twilight Day?