You might find it easier to use iCal's Apple Events scripting interface. If you don't like the idea of using AppleScript directly, there is Apple's Scripting Bridge or, even better, use Appscript for Objective-C, Ruby, or Python. Here's one way to access and modify the events for a given day using py-appscript:
>>> from appscript import *
>>> from datetime import datetime
>>> start_time = datetime(2009,10,26)
>>> end_time = datetime(2009,10,26,23,59,59)
>>> calendar = app('iCal').calendars['Home']
>>> for event in calendar.events[(its.start_date >= start_time).AND(its.start_date <= end_time)]():
... event_properties = event.properties()
... print event_properties[k.start_date], event_properties[k.summary]
...
2009-10-26 18:40:00 <event 1>
2009-10-26 23:38:00 <event 2>
>>> event.summary.set(to='something else')