Your "cake day" is the anniversary of the day you joined a site. These are probably most well known from Reddit, but are also supported in some other internet softwares, or from mods.
My experience, mostly from Reddit, is that cake days do nothing but generate noise - with potentially several people posting off-topic replies saying "happy cake day" and then the person who's cake day it is saying "thanks". This noise usually doesn't get deleted either, so, a day, a week, or months later those posts will still be there.
I assume that some people must get a momentary buzz from getting congratulated for registering some number of years ago, but it's not like it's a birthday. The downsides seem to be to strongly outweigh any slight benefit.
Does anyone have experience from a community they run where they've been able to turn cake days into a net positive, where it doesn't just produce noise, and where it's actually a meaningful date for the members, one they actually remember and anticipate before it arrives?