This is a recent hint from Macworld’s Mac OS X Hints blog.
There are three views in iCal – Day, Week and Month. In both the Day and week view, you can easily adjust the hours that are visible. Think os it as zooming in or out on your schedule.
Simply hold down the option key while in the Day view (or Week) and scroll with a scroll wheel mouse or trackpad.
This is great if you have a full schedule and need detail or if you have things going on late into the day.