Google Maps offers drive time in current traffic and a view of traffic conditions on a given day, but unfortunately not drive time on a given day. It isn't automatic, but you can do some planning by seeing what the traffic would be like on a given day on possible alternate routes.
Updated 2014-06-21 - now features new and old Google Maps
Head to new Google Maps and go to where you want to view traffic.
Clear the search box (yeah, the UX people were out that day).
Hover your mouse over the search area and underneath "Getting around" should appear with some options. Select "Traffic":
You can then select "Typical Traffic" and change the date/time of the traffic you are looking at:
Check out the map and try some different routes - avoid the red bits!
Head to old Google Maps and select your start and finish points as usual. Then select 'Traffic' from the dropdown at the top right:
You should then get a menu on the lower left of the map that allows you to change the date/time of the traffic you are looking at:
Check out the map and try some different routes - avoid the red and black bits!