How do I time discussion boards to show on the dates I want?

There is not an automatic way of doing this in Blackboard prior to version 7.1, but here are some workarounds to control the availability of threads:

1. Don't allow students to post new threads until you want the discussion board to start (control panel, discussion boards, modify discussion forum, unselect "Allow new threads") and/or
2. Lock all threads until you want the discussion to start (get into discussion forum, make sure "show options" is on, select all threads, click "lock"). This means that no one can reply to a thread until it is unlocked.

You will have to remember to go back and manually undo these selections to allow students to start their discussions.