Do full worlds still open at random times (middle of the night) throughout the week or just when Anet does their weekly population calculations? If the latter, do we know when that time of the week is?
Nope. Population is now based on averaged play hours in WvW rather than the number of logged in accounts. The status usually updates once a week during the day (Pacific time), Mondays IIRC.
It's getting updated Tuesdays.