As I mentioned on page 6, iPhoto stores imported shots as events. When you import photos from a camera, iPhoto automatically splits them into events (unless you uncheck the Split Events box in the Import pane).
Normally, iPhoto considers one day’s worth of photos to represent an event. For example, if you import some shots taken over a weekend, you’ll have two events: Saturday’s photos and Sunday’s.
Using the Preferences command, you can change the interval of time iPhoto uses when splitting photos into events. Choose iPhoto > Preferences, then click General. The Autosplit Events pop-up offers you three additional choices.
One event per week. iPhoto creates a new event for each seven-day period. Take a two-week vacation, ...