The Talmud (Megillah 15a) writes that Esther emphasizes that her plan is “not in accordance to the law” to refer to the fact that this plan of voluntarily submitting herself to Achashverosh is not in accordance to the laws of the Torah, which forever forbids a Jewish wife to be with her husband after being with another man consensually (Sotah 2a).
R’ Dovid Feinstein points out that verse uses the vowel patach under the letter chuf (kadaas) instead of the shva (kidaas), which means the Law instead of a law.
The Sfas Emes reminds us of the famous concept that Purim is equated to Yom Kippur. He explains that both holidays share the characteristic that they represent the reversal of what would otherwise be irreversible situations. On Yom Kippur, we are forgiven for sins for which we should be punished, and on Purim the Jews survived when they were supposed to be wiped out. The Sfas Emes continues that Esther here means the “laws” of nature H-Shem established will thus reversed on this day.