Nyx and Veritas have a calendar they maintain:
http://uoex.net/forum/viewtopic.php?f=6&t=6177
For stuff like this, I usually either post something in the forums, or
every once in a while, there's something more spontaneous announced
in game, over world chat.
Nyx also announces all her month long events on the forums, such as the
Saint Patrick's Quest going on now!
http://www.uoex.net/forum/viewtopic.php?f=6&t=11211