Having organized several parties involving mostly programmers before, i can say that yes, doing it on weekday makes event way more fun, because it puts a strong positive filter on those attending.

It's close enough to the weekend so you don't have to worry too much about a cheeky extra beer and at the same time it's fairly convenient for most people to swing by after work.

It's almost always on a Thursday. We've tried alternative days in the past and Thursday meet-ups tend to be the most accessible.