I scheduled Fridays off last semester, and could schedule quite a few of my interviews on Thursdays or Fridays. Monday was also a common option. But it really does vary, if you have time and know what schools you're looking at, you could go to MDapplicants.com and search for people who got interviews to those schools. Many people post when they interviewed, so you could see if certain days seem common for those schools...
I like Fridays/Mondays because you can sometimes stay longer at the school and see the city. Some of my interviewers also seemed to appreciate my desire to stay longer and see the area- they probably took it as a sign of true interest in their program.
On the other side, Mon/Fri interviews are often the busiest. At WashU I had 15-20 other interviewees with me on a Friday, while the current students said Weds usually had five or less. Which means you *may* get more individualized attention on middle weekdays...