Do SSA's go live at midnight?

That's not what I've found in the past -- I've played them late on Monday evenings, depending on when Ghost Falcon "flips the switch" and enables them in the store (which may or may not have anything to do with enabling in-game access, but both have been live at the same time in my previous experience).

It's usually very late, though -- after midnight or 1 AM Pacific time in most cases I'm familiar with.

So never, then?

Be careful quoting times, because we're fairly certain that it's not an automated situation, but a manual one. Jesse (Ghost Falcon) mentioned at the Pummit that he has to log into some system in order to push items live in the store (again, I'm not sure how or even if that affects things like story arcs being accessible to play, although I think it's logical to believe they are at least somehow related).

So it's likely dependant on a human being's actual habits affect the situation, and my experience backs that up -- some nights, as you say, items go live around midnight-ish, but other nights it's been after 1 or even (once) 2 AM, and one time that I recall, I waited until almost 3 AM Pacific time and nothing went live (but did sometime before mid-morning the next day).

That nearly killed ya, didn't it?

EDIT: as expected, the new episode IS live, just after midnight Pacific time.