The date you forgot may be the one somebody else used to measure whether you remembered them.

The date you let slip is often the one someone else was quietly using as a test. Not a cruel test, and often not a conscious one, but people do measure their place in your life by whether you remembered without being reminded. What was a small lapse for you may have registered as evidence on the other side.

Rather than trusting memory here, build a system, since caring and remembering are separate skills and only one can be automated. Put the dates somewhere that will interrupt you. And if you have already missed one, a late acknowledgement that admits the miss usually repairs more than a perfect message would have prevented.