Montgomery: «It’s my 30th anniversary.»
Beckett: «Oh, sir, congratulations.»
Castle: «Thirty years of marriage and you don’t know what to buy her?»
Montgomery: «If you stayed married long enough, you’d know it’s hard to stay original after the first 10.»
Castle: «That’s fair.»
«Castle» – «Pretty Dead»
If you think you might forget your anniversary, probably the easiest solution is just create another contact card in Contacts and use the birthday entry for that date.
The birthday can be shown in the Calendar (it’s a checkbox option in its Preferences). Doesn’t look perfect (and yeah, «Anniversary» is just a placeholder), but works: