This method is fast and also reasonably easy, but you have to deal with any duplicates yourself. That means that for repeatedly importing, you have to take care not to import any anniversaries already imported.
Note that by far the easiest way of preventing duplicates is just always to import into a new calendar and delete the old calendar.
File->
Import...->
Import...
New Calendarto import into a new calendar.
How to import anniversaries direclty into iCal
How to export anniversaries as iCalendar file
How to import anniversaries to my iPhone