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Peter Stansky, The First Day of the Blitz: September 7, 1940 Yale University Press, 07 Nov 2007 - 212 pages "...at 4:14 pm on Saturday, September 7, 1940, 348 German bombers - Heinkels, Dorniers and Junkers - and 617 Messerschmitt German fighters crossed the English Channet into British airspace, forming a block 20 miles wide, filling 800 square miles of sky.
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"Black Saturday", http://www.theatlantic.com/magazine/archive/2008/04/black-saturday/6706/ Saturday, September 7, 1940, “one of the fairest days of the century, a day of clear warm air and high blue skies," as the novelist William Sansom recalled. The first raid ended at 6:10, but two hours later more than 300 additional bombers came for a second attack, which lasted until 4:30 the next morning.