Setting alarms is one option.What I have done is set up a separate todo list called (imaginatively) "Anniversaries" where the entries show in the day view of Agenda and the default warning is set to 14 days and then created an annualy repeating to do for each anniversary in the "real" Anniverssary list.
This technique gives you a reminder in your diary far enough in advance to organise cards/presents etc. and it will be visible until you delete it because it's too late or you organised the card etc.
