A spirit day of Duke East Term II (see Spirit week). It takes place on the Tuesday of the second week.
Legends say it began in 2010. (see Hipster TiPster for more information on the origin)
V-neck T-shirts, skinny jeans (of any color), hipster glasses, etc. You have a lot of options. Google some pictures of hipsters to get a good idea for other stuff.
At West this is dubbed Dress Like Roman Day. This was great until it was 2015 and we were better than that.