The first of April is traditionally April Fool’s Day, when people play practical jokes on each other (but only until 12pm – after that the joke’s on you). Nobody knows whose genius idea this was, but April Fool’s Day has been followed since as far back as the 18th Century in England. If you want to know more about April Fool’s Day and jokes here are some useful bits and pieces:
Wikipedia entry (complete with some links)
Some famous April Fool’s jokes.
Books about April Fool’s Day in the library: we’ve got a few and here they are.