Just throwing in an idea which popped up in my mind while reading this.
Considering the fact that a 24h supporter is kinda useless and it could be easily abused if not controlled properly, I'd rather have a special "welcome theme" the first time you open osu on your birthday singing for you.
I know this is a bit silly, but it could be a nice alternative and some users may find it heart warming (like saying "osu! cares about you <3", or something like that xD).
As a solution for the correct "date" we could add a box into the options to input our birthday. This data will be stored locally on our pc (maybe someone doesn't want to make it public on their profile for whatever reason) and it will be asked just once to avoid abusing it (even if it doesn't really hurt anyone if someone just wants to hear that welcome more often, but I'd rather keep it for the right day only to make it more special
)
Probably this is already a different request, but anyways...