As I have added contacts to my mobile phone, I've taken advantage of the additional fields available to include birthdays and anniversaries (including year of birth or wedding for anniversaries) . These are linked to my email provider and my shared calendar. My calendar is 'synced' over all my and DH's phones, computers, tablets etc. - no chance of missing any special dates, especially with notifications set up to remind us a week before.
I know that technology isn't for everyone but if you use a smartphone, it's worth taking advantage of some of facilities on offer.