If the first three days are foggy, rain will make the lanes boggy.
If it rains on the first day of April, there will be rain for fifteen successive days.
"The first of April, when it is an almost universal custom throughout Christendom to play more or less amiably asinine tricks upon one's neighbor. Of the origin of this custom nothing positive is known." William S. Walsh, Curiosities of Popular Customs (1898), p. 62.
Well, we know it has been celebrated for at least 300 years, for Walsh goes on to relate the machinations of Dean Swift (the author of Gulliver's Travels) and two of his friends, who sat up late on the 31st of March, 1713, "contriving a lie for the morrow". They decided to spread a rumor, via their servants, that a man recently hanged had come to life again and was even now at a local hostelry. Human nature being what it is, "mine host would have his hands full with an influx of curious visitors. Next day, however, Swift records that his colleagues did not come up to their agreement, and thus the scheme had failed."
A favorite prank was sending the gullible on "sleeveless" errands: to the shopkeeper for "pigeons milk" or "hens teeth", or to the bookseller for "A History of Eve's Grandmother", or, like Dean Swift's failed joke, to see something extraordinary. Others are still played to this day, like "Mister, your shoe is untied" ("buckles are undone" in the days of buckles), or "Mom, there's something on your face" ("it's your nose, ha ha ha!"). With the coming of the telephone, shopkeepers who carried tobacco in tins often heard the old chestnut, "Do you have Prince Albert in a can? You do? Well, let him out!" and maniacal laughter as the pranksters hung up.
The media cannot resist either, and so you are likely to see any number of hoax stories today: "The World Has Come to an End (details at 11)" and the like.
Just remember that pranks and jokes can only be played until noon today; after that, the prankster becomes the April Fool:
April Fool's gone past
You're the biggest fool at last;
When April Fool come again,
You'll be the biggest fool then.