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.)
Through the Lens of Torah & Halakhah 
Rabbi Helfgot made an interesting point about the early history of Purim. Purim became a holiday because there was a grass roots effort to establish it is a holiday. The rabbi’s didnt proclaim Purim to be a holiday the people did. It appears to me that Mordechai’s letters are part of this “grass roots” effort, almost like a political candidate canvassing to collect signatures. Having been involved in the historical events, who to better lobby the people for a holiday than Mordechai himself.
It looks too like Mordechai added in the gifts to the poor. Is that right?
Why are the letters important? Isn’t it nice that the letters arrived? Is it possible that corrupt messengers could have stopped them? But that doesn’t feel like the answer you’re going for. I think it’s interesting that Mordechai seems to be able to start this tradition on his own. Was there a bet din involved?
Now that I’m home from megillah reading I’m wondering if the letters being sent has to do with authority being transferred to Mordechai.
I think that it is not necessarily about transferring authority, but Mordechai asserting Rabbinic Authority. Rabbis are typically not comfrotable with ritual that is initiated from the “grassroots.” It requires a much greater effort to work with a community and help carve out a meningful minhag / ritual than to just create whole cloth.
This is one of the challenges that pulpit rabbis face all of the time. My authority stems from the community and not from some outside source – ie. a prophet or the Sanhedrin had divine imprimatur. Therefore, I have to walk the line between molding the community based on my vision and embracing the vision that existed and exists here now.
Mordechai had to do the same thing. He took the communal energy and tried to shape in a direction that he so fit. Not always so easy!
Very interesting. I’ve known rabbis who have really offended communities by just bringing their own vision to a community without listening first. I would be very interested in learning more about conflicts that have been created for you and other rabbis by both bringing to and serving a community.