This
is one of the most popular and entertaining Christmas traditions in Spain.
December 28 is the day when everyone is allowed to play practical jokes and
when it is customary to buy joke articles in Christmas markets. It
is a feast similar to our April Fool’s.
Its
name comes from a religious holiday in honor of the young children who were
slaughtered by the order of King Herod.

Nowadays
the religious aspect has been forgotten and it's a day to play practical jokes.
Even TV shows and newspapers are all about this topic. One of the most
widespread jokes on this day is to stick a figure cut out of white paper on
someone's back (without them realising). The word in Spanish for this practical
joke is an "inocentada.