On which day of the month of Adar were the Jews to be destroyed under Haman's edict?

Answer: The thirteenth

Haman's letters ordered the Jews killed on a single day, the thirteenth of Adar, the twelfth month.

“And the letters were sent by couriers to each of the royal provinces with the order to destroy, kill, and annihilate all the Jews—young and old, women and children—and to plunder their possessions on a single day, the thirteenth day of Adar, the twelfth month.”— Esther 3:13 (BSB)