The biblical Purim differed from how it is observed today
The current practice is that Purim is celebrated as a one day holiday in Jerusalem on 15 Adar as a commemoration of the end of hostilities in the walled city of Shushan, where the battles occurred on 13 and 14 Adar, while the holiday is observed elsewhere on the one day 14 Adar to recall the cessation of the battle after the war on the 13th elsewhere. The rabbis, not the book of Esther, determined that cities that had walls since the days of Joshua were obliged to observe Purim on 15 Adar. In contrast, as I will show, the book of Esther states that the holiday should be observed by everyone on two days, on the 14th and 15th of Adar.
It is possible that the date of the holiday developed in three stages. It was first accepted as a holiday on the 14th only in the villages, as stated in Esther 9:19. “As a result (of the successful defensive battle against the non-Judeans on the 13th and 14th day they engaged in battle) Judeans of the villages, who dwell in villages, make the 14th 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, the first stage of the development of the celebration of Purim. (1) The holiday was celebrated for one day, 14 Adar, by villagers. Verse 18 states that the Judeans battled a second day, both the 13th and 14th; “and rested on the 15th day and made it a day of feasting and happiness.” But as A. Cohen notes on 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. Since this is absent, it seems that the first stage of the celebration of Purim was a single day, on 14 Adar. (2) This celebration was accompanied by sending gifts to one another. No mention is made of gifts to the poor.
The second stage was developed later. Mordecai decreed that it should be observed for two days as indicated in verses 20-22 and 27. Mordecai sent a letter to all Judeans of Ahasuerus’ provinces “to accept upon themselves to keep the 14th of the moth of Adar and the 15th 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 the 14th and 15th of Adar, two days. He states that they send gifts to one another. He does not mention the gifts to the poor.
Still later, for unknown reasons, the practice arose to have a single day of holiday observed on different days in Jerusalem and elsewhere and to differentiate between cities like Jerusalem that were walled during the days of Joshua who must observe Purim on 15 Adar and other cities that observe it on 14 Adar.
 The Hebrew manot, “portions,” does not indicate what the portions were, but the Talmud states it is foodstuff, and since the plural is used, they decreed it should be at least two foods.
 In “The Five Megilloth,” The Soncino Press,” 1952, page 238. The Babylonian Talmud Megillah 2b also notes that the law for the walled cities is absent.
 It should be noted that Mordecai did not say send “portions,” but gifts. It is unclear whether portions and gifts should be understood as synonyms or as two different kinds of things.
 Antiquities 6:13
 The issue is discussed in the Babylonian Talmud Megillah 2b.