They just sent out a postmortem email that states that the variance in traffic over the course of the day is only 10% and therefore there is no such thing as "peak". And then in the next sentence said that they scheduled the maintenance at a non-peak time when the most engineering resources would be on hand to deal with any issues.
This is scaring us -- we rely on Heroku every day have large marketing pushes scheduled weeks in advance. This maintenance will forcefully interrupt any running attached processes, so it really is shocking they are planning this during midday.
Considering the widespread outages they caused this week with a 10am PDT maintenance, and the widespread complaint re: their timing, I'm going to chalk this one up to unvarnished arrogance.