If you don't visit the site on your cakeday, reddit will hold your cake until whenever you return. So the reminder should be worded like, "Your cake will be there when you next visit the site" rather than, "You'd better get over to the site or you'll miss your cake!"
It's not about the cake and all about the karma.
I don't follow -- the site holds the cake, and the cake is the key to the karma.
Not sure if the HN community will appreciate this as much, but this is awesome! I just signed up. Not sure if you already did, but be sure to post this to Reddit (obviously). I'm sure it would get much more traction there. Nice work! :)
So far it got more traction here than on Reddit. I probably chose the wrong subreddit (r/somethingimade).