Not sure when it opens, but it was definitely open at 6:30 am on a Saturday when I was there recently
Will second this, and it was closed at 1030PM when I returned.
I recently used the downstairs security at 630am on a Saturday in December and it took about 15-20 minutes as there were only 2 lines for both doc checker and bags open at the time. Did not check upstairs to see if it would have been the same or shorter.