You can add anniversaries to contacts as well. As a perk, when you add a birthday or anniversary date with year, it notes the age/anniversary # on your calendar (a handy reminder that my nephew turned 27 yesterday so I don't get his age wrong).
Two annoying aspects of that: since everyone has to have their own contact card (whereas I prefer to have couples together but it just doesn't work that way), both people will show on your calendar. Thus you get duplicate anniversaries (e.g. Joe's 33rd anniversary, Pat's 33rd anniversary) for that day. I think that's the case even if you enter Joe & Pat as each other's spouses.
Unfortunately, relying on Contacts means that you either have to keep dead people in Contacts, or enter their anniversary and birthday as annual date events. Plus the year. And then do the math in your head.