I often wonder about phrase origins. I figured that I would look at the saying "as mad as a March hare" today in honor of both the Easter bunny's visit today and the fact that Easter has fallen in March this year, something that doesn't happen very often.
Rabbits and hares are infamous for multiplying, and I'm not referring to their math skills. As March is the beginning of the mating season that lasts several months for these critters, they tend to be highly excitable and sometimes downright violent. This phrase has been in use since at least the 16th century and was used frequently enough in speech that it was included in John Heywood's collection of proverbs in 1546.