Albert Barnes Commentary Esther 4:16

"Go, gather together all the Jews that are present in Shushan, and fast ye for me, and neither eat nor drink three days, night or day: I also and my maidens will fast in like manner; and so will I go in unto the king, which is not according to the law: and if I perish, I perish." — Esther 4:16 (ASV)

Again the religious element shows itself. Esther’s fast could have no object but to obtain God’s favor and protection in the dangerous course she was about to undertake.