My explanation (in the bottommost paragraph of #msg-95809197) for why the two outgoing directors are leaving one day early is the best I can come up with; if anyone thinks an alternative explanation is more plausible, they should post it!
A disagreement on whether the company will remain independent or be sold (suggested by caravon) makes sense to me at this pivotal time for ENTA. I personally was thinking along these lines. The directors resigning a day early brings added attention to the shake-up and overrides the threat of these directors not showing up at the annual meeting.