iPhoto 09's Events feature automatically groups photos based on when they were taken, but occasionally it mixes multiple time windows together during import. Luckily, splitting a single Event into two only takes a couple of steps.
To split an Event in iPhoto 09, do this:
- Double-click the event you want to split apart.
- Select the photo that should be the first in new Event.
- Click Split, or choose Events > Split Event.
The Split button breaks a single Event into two.
iPhoto pulls the photo you select, along with all of the images that follow it, out of the original Event and creates a new one tagged with the dates from the photos, just like any other Event.
If you need to merge two Events together, that's easy, too. Just drag one on top of the other, and iPhoto will automatically combine the images into a singe Event.
I use these tricks together when I need to move a handfull of images from one Event to another.