Re: Whose birthday is it today?
The first "Whose birthday is it today?" suggests, as bhaisahab said, that you already know it is someone's birthday today but you're not sure whose. There may be 30 people in the room, earlier you heard someone say that someone has a birthday day but they didn't say which person. In order to find out, you ask "Whose birthday is it today?" You assume that at least one person will say "It's my birthday today!"
If you didn't know there was a birthday today, but were just curious to see if anyone in the room had a birthday, you would ask "Is it anyone's birthday day?" or "Does anyone in the room have a birthday today?"
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.