Sam Elliott -- I WASN'T Invited to 'Lebowski Fest'

Today marks the 15th anniversary of the release of the "The Big Lebowski" ... but you won't see Sam Elliott celebrating at the upcoming "Lebowski Fest" in L.A. ... 'cause Sam says he hasn't been invited.

Elliott -- who played the cowboyish "Stranger" who narrated the movie -- was out in L.A. yesterday ... when we asked about the Lebowski Fest West -- set for L.A. later this month.

FYI -- Lebowski Fest is kinda like a Trekkie convention ... only with more weed. Tons of stars from the movie's cast have appeared at various fests over the years (including Jeff Bridges) but it seems Elliott's invite must've been lost in the mail.

Still, sounds like he's open to attend ... which is pretty cool.

Abide.