Seersucker Thursday is an annual tradition in the United States Congress in which Senators wear clothing made of seersucker on National Seersucker Day. This light, cotton-based material is traditional in the Southern United States.
The tradition was started by Republican Senator Trent Lott of Mississippi in 1996 who wanted to "bring a little Southern charm to the Capitol" to remind the Senate of how Senators dressed before the advent of air conditioning in the 1950s. The practice was temporarily suspended in 2012 amid Congressional gridlock, but began again in 2014.
While this tradition is an annual event, it is also common to see Congressional staffers don seersucker suits on Thursdays throughout the year.
History of the seersucker suit
Seersucker weave was introduced to the American south, probably through British colonial trade, sometime in the second half of the 19th century. The cotton weave, which originated in western India, became a signature look of the United States in the early 20th century because its light weight and pre-rumpled surface made it ideal for the intense humidity of summer. 
Gregory Peck famously wore a seersucker suit in the movie To Kill a Mockingbird, creating a cliché of how small town Southern lawyers dressed invoked by later actors such as Andy Griffith. The image of a bow-tied, seersucker-suited young man in a boater hat is likewise a cliche image of a recent graduate of elite Northeastern colleges.
History of Seersucker Thursday
In 1996 Senator Trent Lott decided to bring the tradition back. His goal was to show that "the Senate isn’t just a bunch of dour folks wearing dark suits and—in the case of men—red or blue ties". In 2004, Senator Dianne Feinstein decided to increase participation by encouraging women senators to follow the tradition. The following year 11 of the 14 women senators appeared on Seersucker Thursday in outfits received as gifts from Feinstein.
As of June 27, 2012, Seersucker Thursday was announced to be discontinued.
As of May 27, 2015, Senator Bill Cassidy successfully advocated for the return of Seersucker Thursday. Cassidy remarked, "This uniquely American fashion has a storied history dating back to 1909... Mr. Haspel said it best, ‘hot is hot, no matter what you do for a living.'" 
- Seersucker Thursday. US Senate. Accessed 17 July 2009.
- Bedard, Paul (13 June 2007). "Suckers for Seersucker". US News and World Report. Retrieved 17 July 2009.
- Jaffe, Matthew (21 June 2007). "Seersucker-Suited Senators Dress for Success". ABCnews. Retrieved 17 July 2009.