For purposes of a larger deception, the Allies agreed to allow the double agent Garbo to warn the Wehrmacht's Abwehr of the Normandy landings. So just before D-Day, Garbo informed his Abwehr handler that he would have important information shortly after midnight, 6 June 1944. But when he attempted to report the massive Normandy invasion a few hours before the landings began, no one in Germany answered his hail. Released from the burden of being the first warning of D-Day's arrival, Garbo could smugly berate his control. Chagrined, the Germans apologized and told him "to report anything suspicious: until further notice we shall be listening here everyday' said his control; adding, a little surprisingly, 'except on Sundays'
Source: ‘Delusions of Intelligence: Enigma, Ultra and the End of Secure Ciphers’, p185-6