There are several different ways to do it. Assuming you're syncing to an Apple iCloud Calendar, one popular way is to share your iCal with them.

On your iPhone/iPad go to the Calendar app (iCal).
Tap Calendars

Then tap on the on the "i" right of the calendar you want to share. 

Under Shared With: tap Add Person

 Enter the name of the contact you want to share with and tap Add.

That's it!  Now the other person can accept and they can view your newly shared iCal calendar.