The Idea in Brief
The events in question in this portion of the Book of Esther occurred over the Jewish Passover, which was a time for the Passover meal and then Feast of Unleavened Bread, which had immediately followed Passover. While Haman relied on the timing of the divination of dice, or the purim, Esther had banked on the timing of Passover. In other words, Haman hanged on the gallows on the same day that Pharaoh and the Egyptian Army were destroyed in the Sea of Reeds. The timing was inauspicious for Haman, and Esther would have known this as a devout Jewess, which is why she delayed the disclosure to King Ahasuerus by one day (the 17th of Nisan), which was the historic day when God had destroyed the enemy of the Jews (Pharaoh and the Egyptian Army).
Haman announced the proclamation of the destruction of the Jews on the eve of the Day of Preparation, which is the 13th of Nisan (which is the eve of Passover).
Esther 3:12 (NASB)
12 Then the king’s scribes were summoned on the thirteenth day of the first month, and it was written just as Haman commanded to the king’s satraps, to the governors who were over each province and to the princes of each people, each province according to its script, each people according to its language, being written in the name of King Ahasuerus and sealed with the king’s signet ring. (emphasis added)
According to the Babylonian Talmud (Baba Batra, Folio 15A), the Book of Esther was written by Jews for Jews. From this point in the text until the end of Chapter 7, the events in the Book of Esther span the Passover meal on the Jewish calendar (13-17 Nisan). That is, the Law of Moses had commanded the Passover meal and then the Feast of Unleavened Bread. However, Esther invoked a fast instead. The three days of Esther's fast are remarkable because they coincide with the Exodus of the Jews from Egypt. If this hypothesis is correct, then Esther may have chosen three days to fast because that was the time that Moses had told Pharaoh was the time necessary needed to flee from Egypt (Exod 5:3 and Exod 8:27). The following graph depicts the Passover and the Feast of Unleavened Bread in relation to the destruction of the Egyptian Army, which was the apparent "script" followed by Esther. Please click to enlarge.
Esther fasted during the 14-16 Nisan, and prepared her banquet for the king on the late afternoon of the 16 Nisan. (As noted, she and her fellow Jews would therefore not have eaten the Passover nor have partaken of the Feast of Unleavened Bread.) The late afternoon would mark the end of her fast for three days "night or day" according to Esther 4:16. At this very point in the Book of Esther (when she gives the first banquet) appears the following verse.
Esther 5:7 (NASB)
7 So Esther replied, “My petition and my request is [. . . .]
(ellipses added for emphasis)
In the Hebrew text, she does not finish the thought of the sentence. This verse is pivotal, because it occurs in the very center of the Book of Esther according to the endnotes of the Masoretic Text, which is found at the top of Page 932 of the PDF version online. This verse is in the very middle of the book, and so is the literary "tipping point" in the narrative. This verse is also the day that Esther breaks her fast, which coincides with the Festival of First Fruits, when a sheave offering was waived before the Lord in anticipation of his blessing to the Jews. Fifty from this date was when God gave the Law to the Jews at Sinai according to the Babylonian Talmud (Pesahim, Folio 68B). In other words, these very days portended very ominous spiritual power, but in the favor of the Jews. This was the apparent "script" followed by Esther.
Why Esther postponed her disclosures to the King until the following day
If the foregoing paragraphs are correct, then Esther was following the "script" of the Exodus narrative. While there is no explicit mention of God (or even prayer) in the Book of Esther, it is very evident that Esther relied on the literal Torah (the Word of God as divine revelation) as her guide through the darkness of an imminent holocaust event. She met spiritual darkness with the divine power of the Word of God as typified in the Exodus narrative. In this regard, she postponed her disclosures, because the following day was the 17th of Nisan, which was the historic day when God had destroyed arrogant Pharaoh and his army, who were the enemy of the Jews. This logic was the apparent "script" followed by Esther. She did not know what tomorrow would portend, but she followed the "script": that is, the following day would somehow have to be the very day that God would destroy arrogant Haman. And that is what happened.
The Book of Esther revolves around the timing of Passover and its significance for the demonstration of the power of God. This divine power destroys not only the enemies of the Jews, but also defeats the wicked spiritual powers behind them. Haman relied on dice (purim) to divine the most auspicious time to destroy the Jews, and implied here was divination, or reliance on outside wicked spiritual power. The Torah and Psalms are replete with references to defeating "the gods of the Egyptians" during the Exodus narrative, and of course, as many, many scholars have observed, the ten plagues of Moses appeared to be in direct confrontation with Egyptian deities, or wicked spiritual powers. In the same way, following the same "script," Esther's faith had demonstrated that complete reliance and trust in the Torah (divine revelation) was sufficient power to defeat the Jew's enemies and the wicked spiritual powers behind them. Prayer and God are never mentioned in any explicit terms in the Book of Esther, but the "script" of the Exodus narrative is very pronounced. Thus the climax of the narrative "script" was not going to occur on the 16th of Nisan, but on the 17th.