Dawnless Day
From Tolkien Gateway
The Dawnless Day refers to March 10, T.A. 3019, when fumes from Mordor veiled the Sun in an attempt by Sauron to disparage or misguide his enemies. The veil blacked out most of Gondor and Rohan. Though some beams of light could pass through it, it wasn't driven away by a Western wind until the afternoon of March 15, when Aragorn and his companions came to the Battle of Pelennor Fields. Some argue that this wind was made by Manwë Súlimo.