E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Easter Monday, April 14th, 1360, was so called. Edward III. was with his army lying before Paris, and the day was so dark, with mist and hail, so bitterly cold and so windy, that many of his horses and men died. Monday after Easter holidays is called Black Monday, in allusion to this fatal day. Launcelot says:
It was not for nothing that my nose fell ableeding on Black Monday last, at six oclock i the morning.Shakespeare: Merchant of Venice, ii. 5.
February 27th, 1865, was so called in Melbourne from a terrible sirocco from the N.N.W., which produced dreadful havoc between Sandhurst and Castlemaine.
Black Monday. In schoolboy phraseology is the first Monday after the holidays are over, when lessons begin again.