According to Wikipedia, in Scotland the custom was known as "hunting the gawk," i.e. the cuckoo, and April-fools were "April-gawks," the cuckoo being a term of contempt, as it is in many countries. As the old New Years day was April 1st, French and Dutch references from 1508 and 1539 respectively describe April Fools' Day jokes and the custom of making them on the first of April. France was one of the first nations to make January 1 officially New Year's Day (which was already celebrated by many), by decree of Charles IX. All of this was easily summed up in the episode "So It's Come to This" of the fourth season back in 1993 of "The Simpsons," where Lisa explained that the pagans continued celebrating the new year on April and the Christians making fun of them.
In the pilot of the American "The Office," one of the employees put their boss Michael's mug that says "World's Best Boss" in a Jell-O mold. I was planning to do so to my friend who loves "The Office" and has such a mug--for April Fool's Day. He keeps it in his office, under lock and key, the office, not the mug. My accomplice went into the office where his office is with my friend but unfortunately my friend never left so my accomplice could get the mug for me to put it in the Jell-O. My accomplice waited for my friend to leave but he never did. So I never got to do the prank. Oh well.... there is always next year.