I looked up the history of April Fool’s Day, and decided to share a shortened version for you:
The history of April Fool’s Day or All Fool’s Day is uncertain, but the current thinking is that it began around 1582 in France with the reform of the calendar under Charles IX. The Gregorian Calendar was introduced, and New Year’s Day was moved from March 25 – April 1 (new year’s week) to January 1. Communication traveled slowly in those days and some people were only informed of the change several years later. Still others, who were more rebellious refused to acknowledge the change and continued to celebrate on the last day of the former celebration, April 1. These people were labeled “fools” by the general populace, were subject to ridicule and sent on “fool errands,” sent invitations to nonexistent parties and had other practical jokes played upon them. The butts of these pranks became known as a “poisson d’avril” or “April fish” because a young naive fish is easily caught.
My husband is a prankster. He has a fun-loving nature and a soft heart. Twenty-three years ago, on April 1, he 
took me out to dinner at the Turning Point Restaurant in Piermont, NY. During dinner he asked me to marry him, smack dab in the middle of me chewing food, in a tone that I liken to discussing the weather. Now, the back story is that he had asked me before and I had said no. (He had asked four months after we dated, and we Capricorns are slow, cautious spirits that don’t move that quickly.) I nodded yes before I had swallowed my food, eyes widened. How utterly romantic. 
. Then he added, Larry being Larry. “If you said no, I was just going to say: April Fools-ha-ha.” He had it planned, ring and all.