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You need to sync it to some service that you can both access. My wife and I have Google Apps (free) accounts, so we each have one another's calendar visible on our iPhones.
 
On your Mac you can set it up in iCal. They will have to sync it when they connect to iTunes. in iCal click on the events you want to share. Then on the toolbar Click Calendar - Click share. From there you will enter their email addy.
 
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