December Fools' Day

If you visit Spain during the Christmas season, you have to watch your back on 28th December. Instead of April 1st, Spain celebrates El Día de los Santos Inocentes (Day of the Holy Innocents) as their national prank day. The ultimate goal is to subtly tape a white paper stick-figure (un monigote) onto a friend’s back without them noticing, making them the ultimate "innocent" fool of the day.