At 11:29:51 am on Thursday, November 7, 2013, while the 6th Latin American Congress of Photobiology and Photomedicine in the Peruvian city of Arequipa was taking place, locals stopped their activities to contemplate the day without shadow  [es].
This event happens twice a year in cities located between the Tropic of Cancer and the Tropic of Capricorn, that is, between Northern Mexico and Northern Argentina (in America). The next day without shadow for Arequipa will be on February 3, 2014.
On Twitter, Paula Gonzalez warned:
Arequipa: Alta radiación ultravioleta por el Día Sin Sombra http://t.co/cSru3r6CLK 
— paula gonzalez (@pgpaulagonzalez) November 7, 2013 
Arequipa: High ultraviolet radiation on the Day Without Shadow.
Meanwhile, Renzo Vargas explained it with 140 characters:
El dia sin sombra, solo sucede 2 veces al año por unos minutos, ya que el sol se coloca en un angulo de 90°. #quelocura 
— Renzo Vargas (@renzovargas) November 7, 2013 
The day without shadow only happens twice a year for some minutes, as the sun puts itself in a 90° angle. #quelocura  [this is crazy]
With the sun on a perpendicular position, ultraviolet radiation  in Arequipa on Thursday, November 7, reached up to 16 units, considered extremely high.