"And thus I [Esther] will approach the king." (Esther 4:16)
Rabbi Yosef Gackon writes, concerning Esther’s three-day fast, that Esther [in fasting for exactly three days] had the following kabbalistic intention. Namely, that three days and nights contain seventy-two hours, and "B’chen" ["thus", in 4:16] equals seventy-two numerically [in letter-gematria], corresponding to the [exalted 72-letter] Name of G‑d hinted in the three verses [that begin], "Vayisa", "Vayavo", "Vayet" [Exodus 14:19, 20, 21, each of which contain seventy-two letters precisely]. It was with the power of this Name that G‑d split the [Red] sea and had the Israelites cross over, and He guided them in His protection and they had no fear [of the enemy].
Wisdom from our sages across the ages
From that Name, [the flow of] the Divine life-force comes to Esther's supernal [corresponding] ; for Esther [as our Sages say] was greenish in complexion [green is a color associated with the sefira Chesed, Divine kindness, as the life-flow descends through the upper worlds]. Esther, with the power of these seventy-two hours, approached[G‑d], King of the world, in her prayers; for she was sure of His help [that He would answer her and save the Jews]. Then, in this [lowly physical] world, she approached King Ahasuerus.
[Translated by Carmel Kehati from p. 263.]