An impersonal subject
A: Do you know anything about May 16th?
B: It is your birthday.
Here in the dialogue, "It" is an impersonal subject like "It is raining", etc or just implies "May 16th" and a pronoun of "May 16th" like
A: Give me your computer.
B: Why do I have to give it to you?
Thank you so much as always and have a good day.
Hero of the day
Person voted on the most answers.