The Malbim writes that Esther describes the party as one she made in order to parry off any objections to postpone the feast. Since it is made, she intimated, it is ready already.
R’ Yehonasan Eibshutz writes that, since this feast will occur on the 15th of Nisan, it is the Yom Tov of Pesach, and the meal is prepared because she had to prepare it before Yom Tov. In contrast, when Esther invites them to the second feast (Esther 5:8), she does not say she made it ready because that meal could be prepared then without the halachic restrictions of Yom Tov.
As a hint that this occurred on the 15th of Nisan, the M’nos HaLevi points out that the verse, itself, has 15 words.