I think the simplest answer is probably the best. It only makes sense if it happened that particular night. It couldn't have been "one night". It was that night, ie.,, the night after the conversation with his wife, which followed his embarrassing encounter with Mordechai, which occurred on his way home from Esther's feast. It was the night preceding the second feast. After all, following the parade, which occurred the next day, he was whisked away by the royal court to attend that second feast. It was a very busy (and decisive) couple of days for Haman.
I think the language can be analyzed, but any meaning derived from the choice of words have to assume as a given that it was then in the course of things. It is not, as you commented on an earlier answer, a random night. That is simply incorrect in a plain reading of the text.