The Ride of the Rohirrim took place in 3019 T.A. It was an event that turned the tide in the Battle Of The Pelennor Fields.

King Théoden, led a large army of riders to Minas Tirith, where Sauron's forces were laying siege to the city. Though they were heavily outnumbered by Sauron's army of Orcs and Haradrim, they gathered their courage and unleashed their onslaught on Sauron's unexpecting servants. Sauron's forces believed that Rohan had been wiped out by Saruman, so they did not have knowledge that the Riders Of Rohan were coming.

The Rohirrim carved a bloody swath through Sauron's army, and when joined by Aragorn and the forces he gathered after braving the Paths of the Dead, they were able to claim victory on the Pelennor Fields that day.

Sadly though, Théoden King was dead, along with many of the riders of the muster, but because of their valiant efforts, Minas Tirith was saved.
Encyclopedia entry originally written by Sauron_The_Deciever