A little more detail...
Originally Posted by FroukjeMarije
Anniversary Festival is always scheduled near the original go-live date of April 30.
The Tale of the Shipwrecked Mariners is always around Talk Like a Pirate Day - Sept 19
Bilbo & Frodo's Birthday is Sept 22 so the birthday event coveers that date.
Treasure Hunt is random. Turbine opens that up for a few days whenever they decide to do so.
The rest are roughly seasonal, but have come earlier in some years and could come later.
Cener, Ingo, Rilibald, Hesred, Halras, Loringo
Arkenstone (ex-Elendilmir) - The Osgiliath Guard