Yesterday I got a notification for a memory from three years ago called Kris's Birthday. iCloud Photos had somehow found out that it was Kris's birthday yesterday, and that he and I were at the same event on his birthday a few years ago.
I really like these kind of memories and alerts. They're timely, often bring back events I had forgotten about, and the photo selection is quite good.
Only, this time, the event itself had nothing to do with the fact that it was my friend's birthday. And I couldn't find a way to change the memory's title within iOS Photos.
Thanks to @joshducharme, that's now been solved: