No it is when you check in. I am going to ask them when I check in as we dont need to be there at a specific time, although I wont go for it if they offer next day
it depends on the airline and the situation - i've been on many flights where they made a general announcment to everyone sitting at the gate asking for volunteers....
even if you'd already checked in and had your boarding pass you could volunteer to be bumped...
i've never done it, but i've been on zillions of flights where there's been a lot of bumping...
in august, we made a point of getting to the airport early so that we wouldn't be involuntarily bumped as i knew that both our connecting flight and our kids' connecting flight were terribly oversold.
They offered it to me once, but it was on the way to Orlando and I only had 4 days at WDW to begin with so didn't take it.
Would have loved to get bumped up on the way home but they didn't ask.