noun
what day; which day
Asks for a specific day of the month or day of the week. Often used with か or in questions.
今日は何日ですか。
What day of the month is it today?
次の会議は何日ですか。
What day is the next meeting?
noun
how many days
Asks about a duration or number of days. Often used with counters or in expressions like 何日間.
旅行は何日ぐらいですか。
About how many days is the trip?
この薬を何日飲みましたか。
For how many days did you take this medicine?