An expression asking or indicating which month and which day; used to ask for a specific date (i.e., 'what date?').
会議は何月何日に変更になりましたか？
On what date was the meeting changed?
彼の誕生日は何月何日か覚えている？
Do you remember what month and day his birthday is?
申込用紙の欄に『何月何日』とだけ書いてあった。
The application form had only 'month/day' written in the space.