Should be determined preseason.
But, since you didn't, the 17th is probably good. It will give everyone two weeks notice, and will allow all trades to process in time for the next week's games, which start play on a Thursday. Traditionally, it works well.
My league has always used that date, but for some reason this season they made it Thursday, Nov. 2. Waaaay to early. However, since there was only one trade all season (after the draft and before week 1, after Stallworth was traded his owner had too many Eagles and shopped him around), it probably won't make much of a difference in our case. However, it does feel artificially early.