The Malbim writes that the verse emphasizes that the Jews were to be killed in one day to make sure the Jews would not be able to escape.
The Shema Shlomo writes about a Plan B for Haman. Haman considered that, if the Jews should somehow wiggle out of his plans, at the very least, they should only have one day to celebrate.
R’ Hanoch Leibowitz points out that the Torah has laws regarding not killing a mother animal and her child on the same day (Vayikra 22:28). These laws are meant to teach us mercy and concern for others. The reason Haman is considered thoroughly wicked is because he had none of this mercy – he had no qualms with killing mother and child on the same day.