Night is a pretty important time in the play. It's when all the passionate love scenes occur so, night seems to shelter and protects the lovers, while the glare of day threatens to reveal them. In contrast, the heat of the sun makes the young men of Verona irritable and prone to violence and the street brawls occur during the daytime.
We often think of night as both a time for romance and liberation, as well as a time of danger, and the imagery of night and darkness in Romeo and Juliet carries both night's promises and its threats. Hidden in darkness, Romeo and Juliet's love is free from the social rules that would divide them. But danger also lurks in the darkness, and the secrecy of Romeo and Juliet's marriage will prove fatal to them.
"For, now, these hot days, is the mad blood stirring" (3.1.1).