For the last couple years, the first resorts to open have been Nakiska and Norquay, but you probably don't want to make a trip for them.
Just looking back at my pics from this year, had a great day at Lake Louise near the end of Nov (first big powder dump of the year), and by mid-Dec we were enjoying the dumps all over.
I'd say early December is pretty safe to plan a Banff trip. If you get the "big 3" passes you can go where the snow is. Sunshine, Norquay or Lake Louise. Shuttles buses for all are available, so the people that come on your trips all don't have to go to the same mountain.