There are two elements of opposition that we can identify in this excellent speech that introduces us to Friar Lawrence in Act II scene 3 of this excellent tragedy. His speech starts off by describing dawn in opposition to night. Note how dawn is presented:
The grey-eyed morn smiles on the frowning night,
Check'ring the eastern clouds with streaks of light;
And flecked darkness like a drunkard reels
From forth day's path and Titan's burning wheels.
There is a clear opposition between dawn and night. Dawn is personified as "smiling" with its grey eyes, whereas night is "frowning" and "flecked," being compared to drunkard staggering out of dawn's path and the sun.
However, note too the second, and perhaps more profound and significant opposition that is created between how Friar Lawrence characterises night negatively and Romeo in the preceeding scene characterises it positively. Romeo says, in Act II scene 2, that night is "blessed":
O blessed, blessed night! I am afeard,
Being in night, all this is but a dream,
Too flattering-sweet to be substantial.
Thus there exists a second, deeper and more significant opposition between the way that night is viewed by these two characters, that could be argued to foreshadow the tragedy ahead.