El Dia dels Sants Innocents is on December 28th and is the day when friends and family play jokes on each other, effectively the equivalent of April Fools Day in the English-speaking world.
According to some, the origins of the day go back to the pagan festivals dedicated to Saturn, which in Roman times gave more freedom to slaves and were allowed to play jokes on people.
From a Christian point of view, the Sants Innocents recalls the day when King Herod ordered his troops to kill all the "innocent" children under two years of age in order to try and kill the Baby Jesus.
Over time, the word innocent has taken on the meaning of naïve, which it can have in both of Catalan and Spanish, and people play jokes on each other.
Perhaps the most common joke is to secretly pin a newspaper cutout man, known as a llufa, on people's backs and you often see people walking around completely unaware that they've got something on their back for long sections of the day.