What Month
Which Month
noun
You have what month - as in: what month is it? It's １０月, you fool. Get a job, hippy.
なんがつ
The readings are the readings you learned with the kanji. 何 is technically the kun'yomi, but it feels pretty on'yomi-ish if you ask me. The 月 uses the on'yomi, though.
学こうは何月にはじまりますか。
What month do you start school?
それって何年何月何日何びょう、ちきゅうが何回回ったとき？
When did it happen? What year, what month, what day, what time, what second was it? How many times did Earth spin when it happened?
田中さんは何月生まれですか？
What month were you born in, Ms. Tanaka?