Sunday, September 27, 2015
This day is covered in these episodes:
This date is a guess, the events happen either on September 27 or October 4.
Misa makes Ema and Shizuka ride the rollercoaster at least five times to get some material to base her scene on. She also confirms that the day is a Sunday. With what Aoi and Watanabe were talking about before, this is probably the last Sunday of September.
Misa: "[...] sorry for calling you here on a Sunday."