The current practice is to celebrate Purim as a one-day holiday. Cities that were walled at the time of Joshua’s conquest of Israel – most notably Jerusalem – celebrate Purim on Adar 15 to recall the end of hostilities in the walled city of Shushan, where the battles occurred on Adar 13 and 14. Elsewhere, the holiday is observed on the one day of Adar 14 to recall the cessation of the fight after the war on the thirteenth in all other places of Ahasuerus’s kingdom.
Three-stage development of the holiday
The date of the Purim holiday was developed in three stages. It was first accepted as a holiday on the fourteenth only in the villages, as Esther 9:19 records: “As a result [of the successful defensive battle against the non-Judeans on Adar 13], Judeans of the villages, who dwell in villages, make the fourteenth of Adar a day of happiness, feasting, and a good day, and of sending portions one to another.”
Two things should be noted about verse 19, which details the first stage of the development of the Purim holiday. First, the holiday was celebrated for one day, Adar 14, by villagers who fought non-Judeans on a single day, 13 Adar. Verse 18 states that the Judeans in Shushan battled for two days – both the thirteenth and the fourteenth – “and rested on the fifteenth day and made it a day of feasting and happiness.”
But as A. Cohen notes in The Soncino Press “The Five Megilloth” in verse 19, “After this verse we would have expected another verse giving the law of ‘Shushan Purim,’ that those who dwell in walled cities keep the 15th of Adar” as the holiday of Purim. The Babylonian Talmud, Megilla 2b, also notes that the law for the walled cities is absent. Since this is absent, it seems that the first stage of the Purim celebration was a single day, on Adar 14, for villages and walled cities. Second, this celebration was accompanied by the exchange of gifts. No mention is made of gifts to the poor.
Second stage
The second stage was developed later. Mordecai decreed that Purim should be observed for two days, as indicated in verses 20–22 and 27. Mordecai sent a letter to all Judeans of Ahasuerus’s provinces “to accept upon themselves to keep the fourteenth of the month of Adar and the fifteenth of it, every year . . . and make them days of feasting and happiness and of sending portions one to another and gifts to the poor.” And the Judeans “took upon themselves . . . that they would keep these two days.”
Again, two things should be noted: (1) Mordecai made Purim into a two-day holiday and requested the people to accept it as such, and they did. (2) He added the practice of sending gifts to the poor.
Josephus, writing after 70 CE, records that Purim was celebrated on Adar 14 and 15 – two days. He states that the people sent gifts to one another. He does not mention gifts to the poor.
Third stage
Still later, for unknown reasons, the practice arose to observe only a single day of Purim. But cities like Jerusalem, which were walled during the days of Joshua, celebrated Purim on Adar 15, while all other cities observed it on Adar 14.
Summary
In sum, we see that the biblical holiday of Purim underwent changes, and it is practiced today differently than the Torah mandates. We have no idea why this occurred.

Thank you all for sharing your Wisdom!
Thank you Yaaqoub.
Israel
The changes to the holiday of Purim, which have occurred over time, are a testament to the fact that human culture is shaped by a complex array of factors, including historical events, social and political pressures, and the collective imagination of a community. The reasons for these changes may be lost to us, but it is clear that they reflect the ongoing struggle of human beings to make sense of their world, and to create meaning and purpose in their lives. I would describe the human condition as one of constant striving, where individuals and communities seek to preserve and enhance their existence, and to achieve a state of greater perfection. The evolution of Purim, dear Rabbi Israel Drazin, and its deviation from the original scriptural account, is a manifestation of this striving, as the Jewish community has sought to adapt and reinterpret its traditions in response to changing historical circumstances. The fact that we have no clear understanding of why these changes occurred, highlights the importance of approaching scripture and tradition with a critical and nuanced perspective. A rational and enlightened approach to scripture, one that takes into account the historical and cultural context in which it was written, is essential for understanding the true nature of human culture and tradition. The changes to the holiday of Purim, and our lack of understanding regarding their origins, serve as a reminder of the complex and dynamic nature of human culture, and the importance of approaching scripture and tradition with a critical and rational perspective. Best, stephen
A superbly thought-out evaluation. You nailed it.
It is interesting that all the holidays, without exception, were changed by the later rabbis and are no longer practiced as the Bible mandates. Purim is no exception. It is also interesting to note that we are celebrating Purim during the war with Iran. These are interesting times. The Ayatollahs have been in power for 47 years, since 1979. The world is changing before our eyes. Israel’s archenemy is defeated. Khamenei was killed — just as Haman fell 2500 years ago — just in time for Purim. They tried to destroy us, and they failed. Didn’t they learn that lesson 2500 years ago? When will they learn?
You and Stephen are really on top of things.