Today I got an e-mail from a friend thanking me for the cards I've sent to her and her husband for Mother's Day, Father's Day, and their anniversary. She said, "I don't know how you keep track of it all. If it wasn't on Facebook, I'd forget my own birthday!"
Ironically, I am not on Facebook (as of now), and I don't remember when her birthday is. I know it's sometime toward the beginning of February. Whereas Mother's Day and Father's Day are the same for them as they are for my parents (and everybody else in the country). I don't strictly remember when their anniversary is either, but I know it's sometime around the end of May along with at least two anniversaries in my family, so it's easy enough to remember to buy another card and send them all out at once.
I noticed this year that my parents' anniversary is exactly a week before my birthday (which would be quite odd if I were the oldest child, which I am not), so I should be able to remember it from now on.