"Why, this is very midsummer madness!"
-- Olivia, Twelfth Night, Act 3, Scene 4.
"Midsummer madness" was an English term commonly used during the fifteenth through eighteenth centuries for the peculiar kind of madness that strikes lovers during the summertime -- namely the increased tendency to fall in and out of love at the drop of a hat, often for no reason readily apparent to outside observers. Some believed that this madness was brought on by the heightened influence of the Fae during that time; others, the change in the weather. March-mad is a similar term, though it refers to the March hare.
Either way, Shakespeare got a lot of good mileage out of midsummer madness... as have I, for that matter!