You can find them in the announcements of the summer event and the fall eventIs there a topic with more info on limited buildings? I haven't been paying attention to FoE for a while and last I saw something about it were some concept images with some hand written stuff on it.
Forge fountain and cobalt blue lagoon
Both are active for about 5 days and give a certain boost/effect during that time - after that they decay to regular decorations without any real worth behind them