Shakespeare plays with images of light and darkness throughout Romeo and Juliet; indeed, many of the scenes happen either late at night or early in the morning. In act 2, scene 2, however, Romeo provides some of the most famous lines in all of English literature:
But, soft! What light through yonder window breaks?
It is the east, and Juliet is the sun.
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief. (II.ii.44-47)
In these lines, Romeo compares Juliet to the sun. This is after the Capulet party, and Romeo is awaiting a glimpse of his beloved as he waits below her balcony. It is dark, late at night, and Romeo believes that Juliet's beauty is enough to transform the night into day, much like a sunrise.
It's also important to remember that Romeo has been through his own symbolic "night" recently over Rosaline's rejection. In fact, just before this party, Benvolio has to persuade Romeo to even attend, telling him to "Compare her face with some that I show,/ And I will make thee think thy swan a crow" (I.ii.87-88). It could also be argued that Rosaline is the darkness that has encapsulated Romeo's heart, and Juliet is the sun that breaks through that darkness with her beauty.
Romeo goes on to ask Juliet to appear at her window ("Arise, fair sun...") and "kill the envious moon." Here, the moon symbolizes the passions Romeo held before, whose light and beauty pale in comparison to the bright and beautiful Juliet. Romeo realizes that anything he considered love before beholding (and kissing) Juliet is inconsequential.