On Purim there are four mitzvos — commandments — that relate to the day: to hear Megillas Esther twice (evening and morning), to have a festive meal during the day, to give gifts to two poor people, and to give two items of food to one person (minimum).
When reading Megillas Esther there is one obvious omission: the name of G-d. This is the only one of the books that comprise our holy writings (Tanach; Torah, Prophets and Writings) that does not mention the name of G-d.
The Midrash states (Midrash Mishlei) that while in the time of redemption all holidays will vanish, Purim shall remain. Maimonides (as understood by the Ravad) mentions in the Laws of Megillah 2:18, that none of the holy writings will be read in public except for Megillas Esther.
What is so unique about Purim and Megillas Esther that while the name of G-d is not mentioned even once in the writings, nonetheless, Purim supersedes all the other holidays in the Messianic era?
To answer this let us quote from the Talmud in tractate Megillah (6b), which discusses Purim: The Talmud states that during a leap year, Purim is to held in the second Adar so that the redemption of Purim will be as close as possible to the redemption of Pesach, which is in the following month, Nissan.
(The Jewish calendar follows a lunar as opposed to a solar cycle. This means that every Jewish year, we fall eleven days behind the seasonal cycle which works according to the solar calendar. To ensure that the Jewish holidays are congruent with the solar calendar, every two or three years we add an extra month. The month that is added is Adar, the month in which Purim falls, thus leading to the question, in which Adar is Purim observed, the first or second?)
In other words, the crucial element of Purim is identical to that of Peasch: the element of redemption. And by having these two holidays which focus on the redemption of the Jewish people in such close proximity one is able to clearly see the contrast. The redemption of Pesach is on a grandiose scale. People saw the hand of G-d change nature (the ten plagues and the splitting of the sea) and redeem them from Egypt. The miraculous nature of Purim is much more subtle. There are no open miracles, just a series of events which when put together led to the redemption of the Jewish people. And in a certain sense, this is an even greater miracle than the miracle of Pesach!
It is a greater feat for us to see G-d manipulate events within nature, as He did on Purim, than it is for us to see Him manipulate them from outside of nature, as He did on Pesach (obviously, it is all the same from G-d‘s perspective).
And when the messiah comes, and we will have a broader perspective on history, we will see that miracles of Purim, i.e. the ones that happened within nature, were actually quite prevalent during our years of exile. It is for this reason that Purim supersedes all other holidays in the Messianic era, and why the absence of G-d’s name in Megillas Esther actually testifies to His Divine Providence.