This is just a friendly reminder to prepare your bag of tricks and/or be on guard.
The origin of this custom has been much disputed. Many theories have been suggested.
What seems certain is that it is in some way or other a relic of those once universal festivities held at the vernal equinox, which, beginning on old New Year's Day, the 25th of March, ended on the 2nd of April.
It has been suggested that Europe derived its April-fooling from the French. France was one of the first nations to make January 1 officially New Year's Day (which was already celebrated by many), by decree of Charles IX. This was in 1564, even before the adoption of the Gregorian calendar. Thus the New Year's gifts and visits of felicitation which had been the feature of the 1st of April became associated with the first day of January, and those who disliked or did not hear about the change were fair game for those wits who amused themselves by sending mock presents and paying calls of pretended ceremony on the 1st of April. French and Dutch references from 1508 and 1539 respectively describe April Fools' Day jokes and the custom of making them on the first of April.
Though the 1st of April appears to have been anciently observed in Great Britain as a general festival, it was apparently not until the beginning of the 18th century that the making of April-fools was a common custom.
In Scotland the custom was known as "hunting the gowk," i.e. the cuckoo, and April-fools were "April-gowks," the cuckoo being there, as it is in many countries, a term of contempt.
In France the person fooled is known as poisson d'avril. This has been explained from the association of ideas arising from the fact that in April the sun quits the zodiacal sign of the fish. A far more natural explanation would seem to be that the April fish would be a young fish and therefore easily caught. The French traditionally celebrated this holiday by placing a dead fish on the back of friends. Today the fish is substituted by a paper cut out.
The Dutch celebrate the 1st of April for other reasons. In 1572, the Netherlands were ruled by Spain's King Philip II. Roaming the region were Dutch rebels who called themselves Geuzen, after the French "gueux", meaning beggars. On 1 April, 1572, the Geuzen seized the small coastal town of Den Briel. This event was also the start of the general civil rising against the Spanish in other cities in the Netherlands. The Duke of Alba, commander of the Spanish army could not prevent the uprising. Bril is the Dutch word for glasses, so on 1 April, 1572, "Alba lost his glasses". Dutch people find this joke so hilarious they still commemorate the first of April.
Chaucer's story, the Nun's Priest's Tale, written c.1400, takes place on 32 March; that is, 1 April; it is Chanticleer and the Fox, a story of two fools.
Many media organizations have either unwittingly or deliberately propagated hoaxes on April Fools' Day. Even normally serious news media consider April Fools' Day hoaxes fair game and spotting them has become an annual pastime. A number of serious journals would publish hoax articles in their April volumes. The advent of the Internet as a worldwide communications medium has also assisted the pranksters in their work.
Hoaxes can be child-friendly as well. Some fun ideas include decorating the house with "Happy New Year" signs or putting your clothes on backwards"