There's no set date. Roughly mid-December to mid-January, usually coincidental to the Game Updates. So it should start when GU 41 hits, and end when GU 42 hits.
Edit: I see Bludwyng has been on Test getting Frostfell quests into the db :)
YAY! I already broke out the Frostfell decorations for my little Kitty's house in SQ. Can't wait til it goes LIVE... saw the robes on another site, they look great!
as of today, December 12, FROSTFELL IS LIVE!!! so break out your holly and mistletoe, hang your stockings by your hearths, and have a joyous frostfell this year.