If you should be in a Spanish-speaking country today don't believe everything you're being told or read in the paper. As our April Fools' Day is little known in Spain and Spanish-speaking Latin America the rough equivalent is El Día de los Santos Inocentes and is observed on Dec. 28th each year in much the same way as our April Fools' Day.
In its origins, the day had a sort of gallows humor. The Day of the Innocents observes the day when, according to the Gospel of Matthew in the Bible, King Herod ordered the baby boys under 2 years old in Bethlehem to be killed because he was afraid that the baby Jesus born there would become a rival. As it turned out, though, the baby Jesus had been taken away to Egypt by Mary and Joseph. So the "joke" was on Herod, and thus followed the tradition of tricking friends on that day.