'Tis Midsummer-moon with you. You are stark mad. Madness is supposed to be affected by the moon, and to be aggravated by summer heat; so it naturally follows that the full moon at midsummer is the time when madness is most outrageous.
What's this midsummer moon? Is all the world gone a-madding?
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894