Happy New Year
Back in the day, new years was on April 1st. However one day, a Pope decided that Jan 1st was to become the new years day. And anyone who believed that April 1st was still new years day was called a fool. Hence: April 1st becoming April fools day.
New years used to be celebrated over 8 days, and either Pope Gregory XIII, King Charles IX, or his men, changed from whatever calendar they were using to the new Gregorian Calendar. This involved the removal of 10 days during the year period, and somehow moving New years to Jan 1st as well.
Being that information was slow to change back in the day, many of the french still believed that it was April 1st, and therefore continued to celebrate it on that day. Essentially pranks were played on the folks who believed in the old new years day. These pranks caught on, and the tradition spread (in various forms) from France to England and the rest of the world.
Google for more on the longer story. My mind is fuzzy, and I'm pretty damn tired :^)