Rashi on the Prophets and Writings

And Mordecai knew all that had transpired The Master of Dreams told him that the celestial beings had concurred about it, because they had prostrated themselves to an image in the days of Nebuchadnezzar and because they had enjoyed Ahasuerus’s feast.

for one may not enter It is improper to enter the king’s gate dressed in sackcloth.

the king’s orders and his edict When the couriers carrying the letters passed, the edict was given in the city.

the full account of the silver the explanation of the silver.

Do not imagine to yourself Heb. אַל תְּדַמִי, do not think, like (Num. 33:56) “And it will be, that which I thought (דִמִיתִי).” Do not imagine to yourself; do not think to escape in the king’s palace on the day of the massacre; because you do not wish to imperil yourself now to come to the king without permission.

and who knows whether at a time like this And who knows whether the king will desire you next year, which is the time of the massacre.
at a time like this For he was presently in Nisan, and the time of the massacre was in Adar of the next year.
you will attain the kingdom if you will attain the greatness which you now enjoy.

contrary to the law for it is contrary to the law for one who is not summoned to enter. And the Midrash Aggadah [explains]: contrary to the law, for until now, I was coerced [to cohabit with him], but now [I will do so] willingly.
and if I perish, I perish And just as I have begun to go to destruction [by appearing before Ahasuerus without being summoned], I will go and die. And the Midrash Aggadah (ad loc.) [explains]: As I am lost to my father’s house, so will I be lost to you, for from now on, that I am submitting willingly to a heathen, I will be forbidden to you.

So Mordecai passed i.e., he transgressed the law by fasting on the first festive day of Passover, for he fasted on the fourteenth, the fifteenth, and the sixteenth, since the letters were written on the thirteenth day.