It could be cool if you enter your IRL birthday and, through whatever formula CA sees fit, it assigns your an in-game birthday! I've thought about this a lot actually and had considered making this
Stardew Birthday Converter (your post finally gave me the push I needed).
The only issue is that holidays in-game don't line up with their corresponding dates in the real world (ex: Spirit's Eve isn't on Halloween). That's because I tried to align this to the equinoxes/solstices. The number of IRL days to in-game days isn't always even because of spacing, but they're all either three or four IRL days:1 game day.
It's probably obvious that I've wanted to celebrate birthdays/anniversaries in-game for a while now.
I get so sad when Shane's birthday passes and I can, at most, take him to the movies! :( For the player's birthday, maybe you can have a party in the Community Center, and everyone over [X number of hearts] shows up/is invited. (Maybe to avoid overcrowding and excessive gift-giving it could be like Animal Crossing where a few villagers come each year? With the exception of your spouse and probably kids, who would always be there.)