Although the reading of the Book of Esther — the Megillah — on Purim is not commanded anywhere in the Pentateuch, the Rabbis teach that it is binding on us and on every generation that comes after us. The holiday itself is a rabbinic institution, not a Torah commandment, and yet its force has only grown with time.
Purim carries four obligations. Read the Scroll of Esther. Eat a festive meal in gladness. Send portions of food (mishloach manot) to friends. And, most important in the Rabbis' telling, give gifts to the poor (matanot la-evyonim).
Why this emphasis on the poor? The Talmud (Megillah 7a) and the 1901 anthology Hebraic Literature both give the same answer. In the decree of Haman, son of Hammedatha the Agagite — the villain of the Esther story — no distinction was made between rich and poor. All alike were marked for destruction. The decree leveled everyone. So it is proper, the Rabbis say, that when the celebration comes, all should have equal cause to feel joy. The wealthy must make sure the poor have enough to eat, enough to drink, enough to feel like real participants in the festival. A Purim in which only the rich rejoice is a Purim that mistranslates its own text.
The teaching has practical consequences. Maimonides, writing in his Mishneh Torah (Hilchot Megillah 2:17), ruled that it is better on Purim to spend more on gifts to the poor than on either meals or presents to friends. "There is no greater joy," he wrote, "than gladdening the heart of the poor, the orphan, the widow, and the stranger. One who does so becomes like the Shekhinah itself."
The holiday, in other words, is built to remember that redemption is incomplete if it leaves anyone out. Haman tried to annihilate every Jew. Purim answers by refusing to rejoice until every Jew can rejoice.