At some point you know enough people that it's just impossible to track so many different dates.

Well, that's true. But what used to bother me was that people I considered myself to be fairly close to didn't try to remember that day. I didn't expect everyone to do so, not even then.

And I'm a bit unusual in that I have a fairly easy time remembering strings of numbers, if they're not too long, and a date is nothing but a string of numbers. You're quite right that that's not true of most people.