The problems here are:
1. Will anyone actually attend a session on the morning of the last day? (One conference ends in the early afternoon, the other in the late afternoon.)Numbers one and two, it would seem, cancel each other out just a bit; if everyone is going out the night before, the likelihood that they'd make it to a session where I might not be in good shape is diminished. On the other hand, that just means that the serious, sober types are likely to be the only ones in attendance. And their questions can be unfun.
2. Can I go out the night before--probably the most fun night at both conferences--and both enjoy myself and not regret it the next morning?
3. Will gnawing anxiety eat its way through the lining of my stomach over the course of the foregoing days?
I'm always ambivalent about low attendance. My first, purely instinctual reaction is often, "Good!" but that's almost immediately followed by disappointment, especially once I actually get into the swing of my paper (I'm a very good performer, despite being an introvert, and I'm narcissistically in love with the sound of my own writing). Questions, though, are a different matter. I've always had a strong physiological "flight" response when put on the spot before more than two or three people, and if a question is odd, detailed, or about an issue I've never even considered, the presence of an audience means that I sometimes get so overwhelmed that it's hard for me to think a question through and respond intelligibly. I'm getting better at managing this as I grow increasingly confident about both my mastery of my material and my ability to roll with the punches--and I hate being asked no questions!--but even on those occasions when I've been barraged with questions and have done a pretty kick-ass job of responding, my heart remains in my mouth until the clock runs out on the session.
So, whatever it is that I'm learning to do to calm myself down during my actual sessions, I think it means I've mostly got number three under control. I'd rather go sooner than later, but as long as I've practiced my paper and thought through some likely questions in advance, I can probably put it out of my mind for a few days. And the advantage to presenting late is the opportunity to get a good read on the attendees, see a few crappy papers (which, when they're given by semi-established scholars, is always awesome for the self-esteem), and even discuss my work ahead of time with potential haters.
All that being said--if the conference gods decided to change those schedules? I wouldn't be sorry at all.