The Dawnless Day

In preparation for his assault on Minas Tirith, Sauron caused Mount Doom to send out a great fume. Overnight it spread westwards across the mountains of Ephel Dúath to cover the eastern lands of Gondor, so that when the Sun rose on 10 March it could not be seen from Minas Tirith, and the lands remained covered in darkness. Thus the night-eyed Orcs of Sauron's armies were freed from their fear of the Sun, while the people of Gondor were filled with dread.

Though 10 March was the Dawnless Day, the darkness that it brought lasted for five further days, during which the people of Minas Tirith endured the invasion of their land and the siege of their city. It was not until the morning of 15 March that the Sun was seen again and the darkness withdrew, though it seems that Sauron had intended for the Storm of Mordor to remain in place even longer than this.