I think an admin should get a small team together for prelaunch and make a bunch of posts so that there is a lot of content on the forum. Then, when it comes time for launch day, you start promoting the day of the launch.
Yes start your promotion the day it launches. Making a big deal about the launch is a great way to draw in participants and build a fan base. You will of course need to promote through the entire life of the forum, but you will likely not get a boon of followers as large as your grand opening.