Although the experiment began on 9/19, 9/23 is described as "Day Six."
When Officer Palmer attends regarding the call about screaming in the basement, it's clearly late at night. However, when Palmer is lifting the sheet in the basement and looking puzzled, the window above the sheet appears to be showing daylight coming in from outside.
Officer Palmer testifies that he attended the call about a woman screaming in the basement of Harold's house, with his gun drawn. Prior to his testimony, when he attended the house, he did not have his weapon drawn.
It's highly unusual that Officer Palmer was dispatched alone to a call about a woman screaming from Harold's basement. He could hardly investigate the matter without potential harm to his safety..
One of the characters identifies September 19, 2014 as a Saturday. It was a Friday.