On page 2B3 of artscroll gemara:
Artscroll says that the residents of Shushan were miraculously granted an extra day to destroy their oppressors. What was the miracle? Wasn't the miracle that they turned things around to their benefit, not that they were granted an extra day to fight their oppressors?
Avraham Black, Har Nof, Jerusalem, Eretz HaKodesh
The Gemara in Megilah (16b) explains what the miracle was. The Gemara cites the verse in Esther (9:12) that relates that King Achashverosh said to Esther that in the capital city, Shushan, the Jews had killed a lot of people. Rebbi Avahu commented that this teaches that a Mal'ach came and slapped the king on his mouth. Rashi (DH Melamed) explains that the king started by speaking angrily (by implying that the Jews had already taken sufficient victims) but suddenly shifted his attitude and asked Esther what her request was. Something dramatic must have happened to account for the king's change of attitude. What changed his attitude? Rebbi Avahu explains that it was the Mal'ach who silenced the king. It was only because of this miraculous event that the Jews in Shushan were able to fight on the following day, for Achashverosh otherwise would have issued a royal order that the Jews must cease their military activities immediately.
Dovid Bloom