They release the info as it becomes available just before (36-48 hours) launch. The actual launch is almost always within 2 minutes of the opening of the window.
Vandenberg is a North to South orbit site. They have to launch when the lines are in place. There are only so many slots to put a polar satellite where it won't conflict with an older one nowadays. Those lines of longintude (pole to pole) pass along as the planet turns.
If you need a spot on the edge of dawn, you must launch then. (very popular time) If you want to see the dark of space, you launch at night (3:40 to 3:55am is a winner for research devices).
If you explore the site, they give you links to each launch live as it happens, usually with video feed, but not until 2-3 hour before the launch happens.
Just save it, and check every so often. When it happens, it goes.
Forget politics, press, or anyne else. Its total science, and nothing can stop the motion of the planets. When the tools are ready, and the stars are aligned, the magic occurs. As it always has Been.
