J. M. Pressley of the Shakespeare Resource Center describes a common reading/staging of this line:

Keep in mind that Romeo, until this point, has merely been addressing a light in a window. This is the point in the speech at which Juliet actually enters the scene. Romeo is both surprised and besotted when young Juliet appears. Rhetorically, Shakespeare is using parallel repetition and alliteration to reinforce Romeo’s emotion.

https://youtu.be/CDa5dLDEQio?t=20s

“Carnival” in this context suggests the festival that precedes Lent, celebrated especially in areas that are predominantly Catholic. This is the festival season that includes Mardi Gras. It traditionally includes elements such as masks, costumes, street parties, and circus-like events–presumably the reason the Good Samaritan is “dressing” for “the show.”

Carnival is an occasion for drinking and cutting loose, a riotous overturning of the normal order of life. Here in Desolation Row it seems particularly colorful and fantastical, but also seedy and ominous: notice it’s the virtuous Good Samaritan getting ready to indulge his wildest urges.

Carnival, Rome, c. 1650

One of the most significant lines in the play.

The word “done” and its variants (e.g. “undone”) appear 34 times in the play, often at crucial turning points in the plot–for example, when Macbeth is pondering the murder of Duncan:

If it were done when ‘tis done, then 'twere well
It were done quickly…

There is even an echo of “done” in “Duncan” and “Dunsinane” (the forest that later becomes integral to the plot), as well as in “dun” (see “pall thee in the dunnest smoke of hell”).

These repetitions toll like bells, giving the impression of a series of fated actions–actions which, as Lady Macbeth despairingly says, are irreversible (“cannot be undone”).

This is the second use of “done” in the play (the Witches used it first). The word and its variants (e.g. “undone”) appear 34 times in the play, often at crucial turning points. There is also an echo of “done” in “Duncan” and “Dunsinane” (the forest that later becomes integral to the plot), as well as in “dun” (brown; see “pall thee in the dunnest smoke of hell”).

These repetitions reinforce our impression of a series of fated and irrevocable acts–our sense that, as Lady Macbeth later says, “What’s done cannot be undone.”

Here is the first of those acts: the promotion of Macbeth to Thane of Cawdor, which sets the whole plot in motion.