http://revach.net/article.php?id=4427

Section: Moadim   Category: Megilas Esther
Rav Yehonoson Eibshitz - One Appearance And Two Parties Later, Why Did Esther Keep Pushing Off Her Confrontation With Haman?
Esther risks her life to go speak to Achashveirosh without an official audience.  She invites him to a party where she says nothing other than let's do it again tomorrow.  At the next party she finally ends her silence and tells here story.  Why did she do this at the second party and not the first?  Was this part of her master plan?

Rav Yehonoson Eibshitz in Yaaros Dvash explains that in order to have a great Nes you need great Siyata Dishmaya.  The Medrash says that when Esther went to Achashveirosh, as she passed the Avodah Zara in the corridor she felt the Shechina leave her and she knew she was in trouble.  Therefore instead of telling Achashveirosh what it was that she wanted, she invited him to a party in her quarters that were clean from Avodah Zara.  There she could merit a special Nes.

However when Haman came to the party with his avoda zara around his neck, she realized she still cannot do anything.  It was only at the next party when Haman was rushed in with his dirty clothing, after walking around town with Mordechai on the horse, that the room was totally clean and the stage was set for the great Nes that ultimately took place.

This is why, says Rav Yehonoson, the Megila makes a big deal about how rushed Haman was to get to the party.  He didn't have time to put on his Avoda Zara that he surely didn't have the chutzpa to where while playing driver to Mordechai at the King's behest.  This was a great Nes in itself that made the outcome possible.