The Dawnless Day
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.
Along with the Darkness, a host from Minas Morgul set forth toward Minas Tirith, as witnessed by Frodo, Sam and Gollum who passed the Cross-roads. Later Orcs from the Morannon took Cair Andros and passed into Anórien.
The Darkness was also observed by the Rohirrim who were mustering their Riders and rode from Harrowdale.
Faramir was rescued by Gandalf and Imrahil outside the Great Gate of Minas Tirith.
Aragorn II led the Dead Men and the Grey Company, across the river Ringló.
The Darkness was eventually driven away by a Western wind (perhaps sent from Manwë) on the afternoon of March 15, when Aragorn and his companions came to the Battle of the Pelennor Fields.