what date?; when?
Used to ask for a specific calendar date (month and day). Often appears in forms, schedules, or when confirming an event date.
What date is your birthday?
What date is the next meeting?
いつ is a general 'when' and can refer to any time, while 何月何日 specifically asks for a calendar date.
Compound of 何月 (what month) and 何日 (what day), forming a fixed expression for asking a specific date.