The graph above is years ago But it is especially important today.
Interestingly, in leap years, about 10% fewer babies are born on February 29th. I think it would be wonderful if your birthday was on a leap day. But I think most people who aren't nerds prefer the less „weird“ days before and after.
There's a lot of good stuff in the link above.Highly recommended read it all.
Since then, we have improved Stan to allow it to fit and improve birthday time series decomposition models using full Bayesian inference.
Here is Aki's birthday case study with all the details. This will also be featured in the Bayesian Workflow book.