It really depends on how large of a launch it is. If you really want to drive sales to this site you want to get your list excited about the up coming release. Start promoting a week or so in advance, you could even do a couple of weeks if you like.

Make sure your list knows this is something special and that they need to get involved as soon as the site goes live. As soon as the site does go live make sure they know now is the time to strike.

You then want to follow up a couple of days later with another email for those who have put the purchase off. Remind them they need to act fast and give them a reason to do so.