An April fool was the victim of a practical joke — the term apparently originated in England in the late 17th century. The term “practical” distinguishes it from a joke made using words.
Milton Wright, in What’s Funny— and Why published in 1939, wrote “The humor of the practical joke lies in contemplating the bewilderment, embarrassment, or discomfiture of the person upon who the joke is played.” The book analyzes humor with plenty of examples. Although long out of print it’s worth searching it out in the used book market.
You can see some 1939 tricks at http://hoaxes.org/af_database/display/category/1939aprilfools .
Jacky Benny’s radio show “April Fools Gags” aired on April 2, 1939. Hear it by going to https://www.otrcat.com/happy-april-fool-s-day-old-time-radio-shows and scrolling down to his name.
Here’s a real-life account of a 1939 April Fool’s Day joke via Reader’s Digest: