In Targum on Megilas esther, it translates ???? ???? ?????? as “????? ?????”, which would correspond to Nisan 17, the first day of Chol Hamoed (if they kept 2 days of Pesach). Since Mordechai was a member of Sanhedrin, there might have been Kidush Hachodesh during Golus Bavel, and there might have been one day of Pesach. According to Rashi in Megilas Esther, Haman was killed on 16 Nisan. From Rashi in Shas – Maseches Megila, it is not so clear.
Here are the facts, all directly derived from psukim:
Haman’s decree was written on 13 Nisan.
Esther decreed that Jews should fast for 3 days (night and day, or 24-hour fasts)
Esther’s first party was at the end of the third day of fasting.
The second party was the next day.
Mordechai, in taking part in the decree to fast, was ???? the mitzvos of eating ??? ?????, and presumably ????? ????? ????? which is conditional on ??? ????? ????? ?????.
Now, when were the three days of fasting. In order to have three complete days, fasting would need to begin on the 14th, with the 16th as the third day. This would explain Targum. One may argue that the first day was not complete. The calculation of ?? and ????, referring to 70 hours for esther, 72 for everyone else, is a commentary from the Gr”a, which is fascinating, but it is not the posuk. The custom is to consider the second day of Pesach, Nisan 16, as Haman’s Yahrtzeit. Many people sing ????? ???? as part of ?????? at the Yom Tov meal. Others drink an extra sip/cup of wine to take note of Haman’s death.
May we always be zocheh to celebrate the demise of our enemies.