Importing to iCal directly to iCal is comfortable and easy but may be slow for lots of anniversaries (if you run out of patience, try importing via iCalendar file).
BirthdayScanner X uses the official Apple iCal interface to "talk" directly to iCal. Unfortunately, this interface is everything but blindingly fast. But as long as you do not have more than some hundred anniversaries or so, it should be fine. (The exact number depends on the speed of your machine and your patience).
When importing into an existing calendar, BirthdayScanner X will automatically update old anniversaries and prevent duplicate events in iCal.
Import to iCalbutton...
FAQ: Why is the import to iCal so slow?
How to import anniversaries into iCal via iCalendar file
How to import anniversaries to my iPhone