The day is just starting where I live, and you can be sure that a great many of well planned out pranks will be happening all day. My very favourite April Fools joke was done by Jonathan Swift, writer of Gulliver's Travels and A Modest Proposal. Then, April Fools was called All Fools Day. In 1708, London's most famous Astrologer was John Partridge, although his predictions were always vague and rarely came true. Early in that year, a new astrologer appeared, called Isaac Bickerstaff. Writing a short almanac for that year, unlike most astrologers, Bickerstaff's prediction was very specific: John Partridge would die on the 29th of March, and it would probably be in his best interest to get his affairs in order before that time. Bickerstaff was, of course, a pseudonym for Jonathan Swift. On March 30th, an anonymous Lord wrote a letter that circled around London, detailing how Partridge had died the day before, admitting himself to be a fraud and that Bickerstaff was the true prophet. On April 1st, everyone believed that Partridge was dead. Church bells rang, mourners and funeral workers showed up at his door, and he was actually legally considered dead from that day on. Any of his published works assuring others that he was alive were dismissed. When meeting new acquaintances, Partridge would have to work hard to convince them that he was indeed John Partridge, and was not dead. For the rest of his life, he was seen as either dead or a walking joke. In his life time, he never learnt the true identity of Isaac Bickerstaff.
You can read more here. Happy April Fools Day!