The most frequently used literary device in Juliet's soliloquy in Act 3, Scene 2 is personification.
Personification is found in the opening lines in which she uses references Greek mythology to personify the sun. When she says, "Gallap apace, you fiery-footed steeds," she is referring to the sun as Phoebus Apollo's horses and chariot. According to myth, every evening he drives his chariot, which is the sun, across the sky, which represents the setting of the sun. Hence, in this line, in accordance with Greek mythology, Juliet is personifying the sun.
Juliet also spends a great deal of time personifying night in this speech. One instance is in the line, "Spread thy close curtain, love-performing night." In this line she is personifying night as one who can close a curtain, the curtain representing the dark shade of nighttime.
Another instance of personifying night is in the line, "Come, civil night, thou sober-suited matron, all in black." This line is not only personifying night because she is asking it to come as if it were something she could command, but also because she is referring to it as a mourning matron dressed all in black. It should also be noted that "sober-suited" is a phrase of alliteration, because the consonant "s" sound is repeated.
Another line of personification in which she beckons night is, "Come, gentle night, come, loving, black-brow'd night." This is also personification because she describes night as having black eyebrows, like a human.
A form of parallelism called asyndeton is found in the line, "Come, night; come, Romeo; come, thou day in night." Asyndeton eliminates the use of conjunctions to create a simple and strong effect, as in the sentences: "Been there. Done that. Bought the t-shirt" ("Schemes," web.cn.edu).
A form of repetition called symploce is seen with the repetition of the word come at the beginning of many phrases, such as "Come, night; come Romeo."