Did you know that if iPhoto doesn't divide your events where you want, you can merge two or more events or split a single event into two events?
To merge events, select the events in the iPhoto window by Command-clicking on them and selecting Merge Events from the Events menu or clicking the Merge button on the Toolbar. You can also use a contextual menu - right-click on the selected events and select Merge Events from the contextual menu.
Splitting an event is just as easy as merging events. Display all the photos in an event and select the first photo you want in a new event. Next, click the Split button on the Toolbar.
You can quickly and automatically split an event that contains photos from more than one day by selecting the event in the iPhoto window and selecting Autosplit Selected Events from the Events menu. The photos will be split into events according to the dates on the photos.
That's all there is to merging and splitting events. So now it should be easy to organize your events in iPhoto '08.