Purim falls on the fourteenth of Adar in an ordinary Hebrew year, the date this rule records, commemorating the deliverance of the Jewish community of the Persian Empire from a plot to destroy them, a narrative preserved in the Book of Esther, read publicly from a scroll, the Megillah, as the festival's central textual practice. The story's villain, Haman, sets the date for the planned destruction by casting lots, purim in Hebrew, which gives the festival its name, and the fourteenth marks the day after the community's successful defense.
What complicates the date is the Hebrew calendar's own structure. Because the calendar is lunisolar, seven years in every nineteen year cycle carry an intercalated thirteenth month, a second Adar, added specifically to keep the lunar months aligned with the solar year required for Passover to fall in spring. In those leap years, Purim moves to the second Adar rather than staying on the fourteenth of the first, a shift covered separately for the Adar II leap-year case, so the fourteenth of Adar recorded here applies only in the more common ordinary years when the calendar carries a single Adar. Purim's practices beyond the Megillah reading, giving gifts of food to friends, giving charity to the poor, and holding a festive meal, are all named in the Book of Esther's own closing chapters as instructions the community set for itself once the danger had passed, making the holiday one of the few in the Hebrew calendar whose customs are prescribed inside its own founding text rather than added by later rabbinic tradition.