What does this quote from "Romeo and Juliet" mean? Who said it? What scene and act is it from?"When he shall die, take him and cut him into...

It is ironic that Juliet here calls the sun "garish" since in Act II Romeo compares Juliet to the sun as he, too, uses light imagery for his love:

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,/That thou her maid art far more fair than she (II,ii,61-66).

Since Juliet's lines are farther along in the play, this reversal of light/dark imagery--imagery that is prevalent throughout the play--is indicative of the changes in conditions for Juliet and Romeo.  For, the lovers, who secretly married in the night are safer during the dark hours and must be clandestine about their actions.