In-Game Calendar is showing my vampires' "birthdays"

[oshizu]

I noticed today that my in-game calendar is showing birthdays for my vampires for the first time.
This confuses me since their age bars were reset upon transforming into a vampire.

Would you know what happens to a vampire on their calendar birthday? Nothing, if I don't use a cake?
Calendar birthdays for my human sims seem to signal when they're ready to enter the next age stage.
For vampires, do they only serve to inform me that my immortal vampires are another calendar year older?

[wa-wa-world]

 I've got this too, when they get to their birthday the date on the callender just moves away again by however many days the lifestage lasts