"On a clear, moonlit night, Flight Lieut. Tom Lane steadied his Halifax bomber as he and his crew approached Krefeld, a city in western Germany. This was the night of June 21, 1943."
705 aircraft participated - 262 Lancasters, 209 Halifaxes, 117 Sterlings, 105 Wellingtons, 12 Mosquitos. 44 aircraft were lost - 9 Lancasters, 17 Halifaxes, 9 Sterlings, 9 Wellingtons - 6.2% of the force.
The raid was carried out before the moon period was over and the heavy casualties were mostly caused by night fighters. 12 of the aircraft lost were from the Pathfinders: 35 Squadron lost 6 of its 19 Halifaxes taking part in the raid.
The raid took place in good visibility and the Pathfinders produced an almost perfect marking effort, ground markers dropped by the Oboe Mosquitos being well backed up by the Pathfinder heavies. 619 aircraft bombed these markers more than three quarters of them achieving bombing photographs within 3 miles of the center of Krefeld. 2,306 tons of bombs were dropped. A large area of fire became established and this raged, out of control, for several hours. The whole center of the city - approximately 47% of the built-up area, was burnt out. The total of 5,517 houses were destroyed, according to the Krefeld records, was the largest figure so far in the war. 1,056 people were killed and 4,550 injured. 72,000 people lost their homes; 20,000 of these were billeted upon families in the suburbs, 30,000 moved in with relatives or friends and 20,000 were evacuated to other towns.
Note: Re: the 6.2% of aircraft lost. That figure does not include a/c that crashed - maybe even written off - in England. Either in training accidents, or Operations. Because, they were not considered to be "lost".