I tried trashing a little test event. In FCP X, I control-clicked on the library where that event resided, and chose "Reveal in Finder"; then in the Finder, control-clicked to "Show Package Contents". Sure enough, there was a folder called __Trash, containing my deleted event. Then I quit FCP X and the __Trash folder vanished.
So if you want to recover a deleted event, you will probably need to do it before you quit FCP X.
Now as to actually recovering it, it seems not to be completely straightforward (unless you can undo back to the point where you deleted it, but then you might call it straightbackward... couldn't resist :-)).
Here is a way that may work (no guarantees, of course).
0) Make a duplicate of your library, just in case the following goes wrong somehow.
1) make a copy of the event from the __Trash folder and store it elsewhere.
2) quit FCP X
3) In the Finder, control-click to "Show Package Contents" and place the event folder you copied in step 1 in the appropriate location within the library
4) move the .flexolibrary outside the library package (if all goes well, you may delete this file later)
5) start FCP X again. Hopefully, everything will be restored.
A little explanation regarding step 4: the .flexolibrary contains the database that tells FCP X what is in the library. Just moving the event folder back will not update the database; that is why if you omit this step you won't see the event, even though it is there. If this file is not present, FCP X will reconstruct it from the package contents.