I used to get really upset that my Dad would not remember the exact day of my birthday - it would be in the right month and mostly in the right week, but would sometimes be off by days.
So, I finally a few years ago figured out how to avoid this. Now, EVERY year for Christmas (in addition to other presents) I ALWAYS get him a desk calendar. The big deal is that I go through it and put in all the important dates in large red magic marker. Birthdays and anniversaries. He also uses it to note his many doctor appointments. He has told me again and again it is the best idea!