Here's a breakdown of when you can find all event items on Ultima. For more details and images of server-exclusive weapons, see this page.
This page will probably evolve and be updated as more items are released. Please help me to find errors or missing info by commenting below.
Special thanks to Cyane for the artwork.
Valentine's Day (February) http://i.imgur.com/Qg8FdnV.png
Easter (April) http://i.imgur.com/Pzv1ls3.gif
Summer (July) http://i