Both sentences are grammatically correct, but they convey slightly different meanings:
1. "It's the night when the Quran first sent."
This sentence implies that the Quran itself sent something. This personification suggests an active role on the part of the Quran, which might not be the intended meaning. It could be interpreted as if the Quran itself initiated the action of sending, which is not the case.
2. "It's the night when the Quran was first sent."
This sentence correctly uses the passive voice and suggests that someone or something else sent the Quran. It conveys the idea that the Quran was sent by a sender, likely referring to the divine act of revelation through the Angel Gabriel to the Prophet Muhammad. This is likely the intended meaning in the context of discussing the night of revelation.
So, the second sentence, "It's the night when the Quran was first sent," is the more appropriate choice for conveying the intended meaning.