There are several ways to delete an Event from your iPhoto library.
WARNING: Deleting an Event deletes all the photos contained in any slideshow, projects such as books, cards, and calendars, and published albums. You can retrieve your photos from the iPhoto Trash until you empty the Trash, which deletes your photos permanently. To empty the iPhoto Trash, select it in the Source list, and then choose iPhoto > Empty iPhoto Trash. (If you’re in full-screen view, move your pointer to the top of the screen to make the menu bar appear.)
To delete an Event:
Select the Event, and then do one of the following:
Choose Photos > “Move to Trash.” (If you’re in full-screen view, move your pointer to the top of the screen to make the menu bar appear.)
Drag the Event to the Trash icon in the Source list. (This option is available only in standard view, not in full-screen view.)