בַּחֹדֶשׁ הָרִאשׁוֹן ... הִפִּיל פּוּר הוּא הַגּוֹרָל לִפְנֵי הָמָן
מִיּוֹם לְיוֹם וּמֵחֹדֶשׁ לְחֹדֶשׁ שְׁנֵים־עָשָׂר הוּא־חֹדֶשׁ אֲדָר׃ (Ester 3.6)
In the first month ... pur—which means “the lot”—was cast before Haman concerning every day and every month, [until it fell on] the twelfth month, that is, the month of Adar.
I understand that the lot is used when one is not sure of or doesn't know the result. So says Rashi:
הִפִּיל הַגּוֹרָל בְּאֵיזֶה חֹדֶשׁ יַצְלִיחַ:
He cast lots to determine in which month he would succeed.
מִיּוֹם לְיוֹם. בְּאֵיזֶה יוֹם בַּחֹדֶשׁ יַצְלִיחַ:
For a specific day. On which day of the month he would succeed
And seemingly Ebn Ezra asks this question (ad loc) - did Haman chose the date himself or was he unsure and let the lot decide?