Haman’s servants cast Pur (lots) to choose the time for the planned attack, and the lot fell on the twelfth month, the month of Adar (Esther 3:7). The casting of lots is presented as the way the date was set in the story (Esther 3:7).
In Esther 3:7, the story pauses to date Haman’s plan with unusual precision. In the first month (Nisan) of King Ahasuerus’s twelfth year, “Pur” (a lot) is cast in Haman’s presence to select a time. The casting runs across options “day to day” and “month to month” until it lands on the twelfth month, Adar, setting the intended timing for what follows. This verse sits inside the opening movement of the crisis (Esther 3), after Haman’s rise and his anger at Mordecai’s refusal to bow. The narrative is building toward an official decree against the Jews, but first it explains how the chosen date was determined.
Esther 3:7 slows the story down to fix Haman’s plan on a calendar. The text explicitly says the casting happens in Nisan (month 1) in the twelfth year of Ahasuerus, and that the outcome points to Adar (month 12). This creates a long lead time between the decision and what Haman intends to do next.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Esther 3:7, then read the surrounding section so the answer stays connected to the passage's flow.
7In the first month, which is the month Nisan, in the twelfth year of king Ahasuerus, they cast Pur, that is, the lot, before Haman from day to day, and from month to month, [to] the twelfth [month], which is the month Adar.