They scheduled it on a Saturday to have it low.
All of the debates being shown by the broadcast networks are on Saturday (except an NBC one in January, which is on the Sunday of MLK weekend, and co-sponsored by the Congressional Black Caucus). The Republicans also have an ABC and a CBS debate in February, which are also on Saturdays. Same with the 2012 debates: The ones done by the broadcast networks always go on Saturdays, presumably because the broadcast networks don't want to pre-empt their normal primetime programming to put on a debate.
Of course, it was the DNC's choice to go with ABC and CBS, knowing that that meant Saturday debates. They could have done more of them on CNN and MSNBC.
