Importing a CSV file of calendar information into iCal will allow you to keep multiple calendars up to date and synchronised. ICal does not support CSV importing by default, but there is a plugin available for this funcyion. iCalTextImport will allow you to export your CSV file using Microsoft Outlook, web calendars and other software and attempts to translate the file into a format readable by iCal.
Download iCalTextImport from the link included in the Resources section.
Drag the iCalTextImport utility to the Applications menu on your dock.
Click "Applications," then click the iCalTextImport application.
Drag your csv file into the iCalTextImport window from its location in Finder.
Click convert to convert it to iCal's native format.
Navigate to the location where you saved the converted file and double-click it to import it into iCal.