Asking for someone’s birthday
To ask for someone’s birthday, you can either use いつ or 何月何日. Both work, however いつ is obviously shorter, so we’ll use that in this example.
おたんじょうび is used in this example to sound more polite, however simply たんじょうび is acceptable as well.
Q: おたんじょうびはいつですか？ When is your birthday?