the term "cake day" just makes me think of r*ddit. I've seen Lemon Day going around, but we probably want to avoid affiliation with lemon party.
not that we can tell anybody what to do anyway, and each instance could even have its own. anyway, it needs to be something that obviously means "anniversary" and doesn't require explanation otherwise it'll just be annoying
the concept of account lifespan duration, regardless of what it's called, exists only to make you feel more attached to your account.