midnight
`` You know the fountain of Kallista in Epirus . Its virtues are renowned . '' `` I have beard of it . '' `` Plunge your scimitar in its midnight waters thrice , on the eve of the new moon , and each time summon the enemy you would desire to meet . He will not fail you . '' `` If you cure the captive , I will credit the legend , and keep the appointment , '' replied Mahomed , thoughtfully . `` I have engaged to do that , '' replied the physician .