On the 13th of Nissan, Achaverosh’s messengers went out with a letter, saying “the Jews should be killed out, man woman, and child, on the 13th of Adar”.
When Mordechai found out about the decree, he sent a message to Esther, that “she should go into the king and petition for her people”. Eventually, Esther replied, that “she would go in”, however, she requested that the entire Jewish Nation should fast for three days and three nights, and with that Zcus (merit) in place, she would go into the king.
As a result, Mordechai, who was the Rosh Sanhedrin, told the Jewish nation to fast on the 14th, 15th and 16th of Nissan -- Even though the 15th of Nissan is Pesach.
What Mordechai did with this decision was abolish a number of the Mitzvahs of Pesach, one of which is from the Torah, namely eating Matzah. Wouldn’t it seem wiser to wait one more day, allow the Jewish people to gain the extra merit of eating Matzah, and then fast the next day? Keeping in mind that there are 11 months from this time until the letter allows the enemies of the Jews to kill out the Jewish nation, there is enough time to wait one more day and gain the additional merit of eating Matzah. Why did Mordechai feel compelled to violate a Torah principle and rush the fast to be right then?