Let's learn about "무슨 요일이야?"

무슨 요일이야 = 뭔 요일이야? : What day is it?

[other expressions]

- 무슨 요일이니? = 뭔 요일이니? (to younger people or friends)

- 무슨 요일이냐? = 뭔 요일이냐? (to younger people or friends)

- 무슨 요일이야? = 뭔 요일이야? (to younger people or friends)

- 무슨 요일인가? = 뭔 요일인가? (to younger people or friends)

- 무슨 요일이에요? = 뭔 요일이에요? (honourific by ‘-요’)

- 무슨 요일인가요? = 뭔 요일인가요?(honourific by ‘-요’)

- 무슨 요일입니까? = 뭔 요일입니까? (formal; more honourific by ‘시'’)

[explanations]

* 뭔 : a short form of 무슨(what)

* 요일 : a day of the week; a weekday

* 이다

* 일입니까? : 일 + 이(the stem of 이다)  + -ㅂ니까?(an  honourific interrogative verb ending suffix)

* a verb + -이니? / -이냐? / -이야? / -인가? : a non-honourific interrogative form of 이다

* a verb +  -인가? + -요 : an honourific verb ending suffix

* a verb + -이에요? / -입니까? : an honourific interrgative form of 이다 by ‘-ㅂ-‘ / '-요'