Phase One à 9:17 on the thirteenth day of the month Adar, and on the fourteenth day of the same they rested, and made it a day of feasting and gladness.9:18 But the Jews that were in Shushan assembled together on the thirteenth day thereof, and on the fourteenth thereof; and on the fifteenth day of the same they rested, and made it a day of feasting and gladness.
The year of the war there were spontaneous celebrations – feasting (mishteh) & gladness (simcha). A great emphasis is placed on drinking through out this book.
Phase Two à 9:19 Therefore do the Jews of the villages, that dwell in the un-walled towns, make the fourteenth day of the month Adar a day of gladness and feasting, and a good day, and of sending portions one to another.
"Therefore – Al kein" implies that this was in a subsequent year. The people prioritized the gladness over the feasting and added two social components – "A Good Day – Yom Tov" & "Gifts to one another – Mishloach Manot." It seems that the people wanted to have this day be a yontif on which work would be prohibited. In addition to adding the issur melakhah (prohibition on labor) they also added gifts to one another.
Phase Three à 9:20 And Mordechai wrote these things, and sent letters unto all the Jews that were in all the provinces of the king Ahasuerus, both nigh and far, 9:21 to enjoin them that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly, 9:22 the days wherein the Jews had rest from their enemies, and the month which was turned unto them from sorrow to gladness, and from mourning into a good day; that they should make them days of feasting and gladness, and of sending portions one to another, and gifts to the poor.
What did Mordechai add or change?
What is the signifigance of sending out letters?
(Thank you to Rabbi Nathaniel Helfgot for presenting this basic structure over Shabbat in shul. Thank you to Ramban in the beginning of Masechet Megillah for fleshing this Out. Thank you to Rabbi Meir Schweigger and the Pardes Pod Cast for presenting this in a similar fashion.)