Mrs D's granddad has only had 21 birthdays, when must his birthday be

On the aniversery of the day he was born of course.

I think there is something missing but if you think outside the box, it becomes evident what they are looking for. Think of a day in February.

Groundhog day... he was born on groundhog day.

You guys are such a good team !

Once every 4 years!!

He was born in a leap year and on leap day.

He was born on the feb 29,which is leap day comes for every four years that is why he has very less birth days