"whose birthday" is correct and can be used in written English.
Here is an example sentence: I invited my best friend whose birthday is next month to lunch.
However, the one whose birthday was being celebrated hasn't expressed any thanks.
MIKLOS HORTHY, Hungary's wartime leader, whose birthday is today, is enjoying a controversial renaissance.
Among them are Mr. Darwish and the young man whose birthday celebration turned into a nightmare.
Bartoli found a memorable way to repay Mauresmo, whose birthday was Friday.
"Quite complex but ultimately flat," is the report from Virginia, whose birthday we are celebrating.
The man, whose birthday was on Tuesday, was not cooperating with investigators, Mr. Kelly said.
It turned out he had another son whose birthday was the same day as Fox's.
Ask to speak with the person whose birthday is closest to the date of the interview.
Also glad to see Santa hasn't forgotten whose birthday it is.
Ochoa dedicated the title to her mother, whose birthday was yesterday.
The entity whose birthday is being celebrated tonight isn't a person.
