There are three explanations for this and all the three are mutually complementary.
- Being a teacher of Jews, Nicodemus might have been ashamed of being seen by others as he was going to a new teacher, Jesus. To avoid the embarrassment he might have come to Jesus “by night”.
- The Jews preferred the quietness of the night for learning the Torah. Jesus is the new revelation from God. Nicodemus come to learn the new Torah from Jesus. Naturally, the best time of the day for it is “night”.
In the Gospel of John, the symbolism of light and darkness is very important (Jn 3: 19-21; 8: 12; 9: 4-5). When Judas went out after receiving the morsel from Jesus, the evangelist says that it was night (Jn 13: 30). Judas was moving from light (from the relationship of Jesus) to the darkness (Jn 13: 2, the satanic power). Then Nicodemus who comes “by night” to Jesus, is moving from darkness to light.