April Fool’s Day – Wikipedia, the free encyclopedia
April Fools jokes must only be made before midday.

I previously thought that April Fool’s Day might have started in France, because they were the first nation to adopt the reformed Gregorian calendar, Charles IX in 1564 decreeing that the year should begin with the 1st of January. But Wikipedia has this to say on the subject:

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 1st of April. This view gains support from the fact that the exact counterpart of April-fooling is found to have been an immemorial custom in India. The festival of the spring equinox is there termed the feast of Holi, the last day of which is the 31st of March, upon which the chief amusement is the befooling of people by sending them on fruitless errands.

Of course, I am reading this Wikipedia entry before noon, which means it might change a lot in the next few hours…