You can split one event into two. Say you have an event containing a hundred shots from an afternoon bike ride and an evening dinner. You may prefer to split the event into two separate, more manageable ones.
To split an event, open it to display its thumbnails. Then, select the photos you want to split off (the dinner shots, for example), and choose Events > Split Event. Finally, name the new event.
Merging events. You can also combine two or more events into one. Maybe you fired off some shots at a party, imported them so everyone could see, then took some more. You want all the party shots to be in one event. Easy: drag one event to the other.
You can also merge more than two events: Shift-click or Command-click each ...