If you should be in a Spanish-speaking country today and play a joke on your friends and follow that up with a shout of "íTontos de abril!" chances are you'll get nothing but blank stares as a reaction. The minor holiday of April Fools' Day is little known in Spain and Spanish-speaking Latin America, but there is a rough equivalent, el Día de los Santos Inocentes, observed on Dec. 28.
The day is observed in much the same way as April Fools' Day. But when the prankster is ready to reveal the joke, the saying is "¡Inocente, inocente!" or "Innocent one, innocent one!" (See the lesson on making nouns out of adjectives.)
In its origins, the day is 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. (This is a sad story to be sure, but according to tradition the babies murdered in Jesus' stead went to heaven as the first Christian martyrs.)