I try to associate certain more special events so that I don't forget my card pins. Birthdays I don't even try to remember because I know I will forget them. I remember two or three, including my own but the rest, the days of close friends, I don't remember. I rely on the memory of others... I don't consider these dates so important. A true friend doesn't mind if you forget to congratulate him, of course, just a true friend!
I remember a rather embarrassing incident almost 40 years ago when someone from school came to register my son and asked me what year he was born. I didn't know. It was horrible for me.

It was unusual to associate your girlfriend's birthday with the day Guy Fawkes tried to blow up the House of Lords (-1). I, if I were you, would have forgotten who I made that association for in the first place.
The best solution is, I think, to write down in a notebook some of the important dates for you... and don't lose it!