A date picker is a control that helps someone choose a day from a calendar. It can be easier than typing a date, especially when a person needs to see which day of the week a date falls on. Think of marking an appointment on a wall calendar.
For example, a booking page might let someone select the day they want to arrive. The chosen date should be shown clearly after the calendar closes, so the person can check it before moving on.
Use a date picker when seeing nearby days is helpful. If a person already knows the date, a clearly labeled text or date field can also work; the important thing is to make the expected date format clear.