B"H
After hearing about the decree, Ester became very worried and she wanted to tell the king right away about nullifying it, but she wasn't allowed to see him without permission, so she asked the entire Jewish people to fast for 3 days right away she she herself fasted then as well, in order to have Hashem's help to see the king at a time she wasn't allowed to
The question is, the decree was made in mid-nissan, for the next adar of the coming year, a long ways away
Why couldn't Ester have simply waited a few weeks or so to be called in to the king at a proper time, then ask him then to nullify it?