E-night tickets are sold 3-4 days before the event. They are only for Disney resort guests and most people purchase them at their resort.
This season they have been holding them on Mondays, Wednesdays, and Fridays. From numerous posts I have read it seems that Wednesdays are the busiest by far. Last year we went on a Tuesday night and it was really crowded. People who went the following Friday said it was quiet. I guess most people think the middle of the week is the best time to go so it gets really crowded.