Second Battle of Winterfell

The Second Battle of Winterfell, also known as the Battle of the Bastards, is a battle late in the War of the Five Kings in which Jon Snow and Sansa Stark attempt to retake Winterfell from Lord Ramsay Bolton, the Warden of the North, and restore House Stark's rule over the North in defiance of House Lannister and the Iron Throne.

It is depicted in the eponymous episode, "Battle of the Bastards".

Prelude
"A monster has taken our home and our brother. We have to go back to Winterfell and save them both!"

- Sansa Stark to Jon Snow

With the death of Robb Stark and most of his bannermen during the Red Wedding, House Bolton is named the new ruling Great House of the North by House Lannister. However, to cement their hold on the North, they betray the Lannisters and secure a marriage alliance through Lord Petyr Baelish between Sansa Stark, formerly the Lannisters' hostage, and Ramsay Bolton, Lord Roose Bolton's recently-legitimized bastard son. With help from the Boltons' prisoner, Theon Greyjoy, Sansa escapes her home during Stannis Baratheon's ill-fated attack on the castle and flees to Castle Black with Brienne of Tarth and Podrick Payne, where she reunites with her bastard half-brother, Jon Snow, who was on the verge of leaving the Night's Watch following his death and resurrection.

Sansa urges Jon to help her retake Winterfell from the Boltons. Jon, wary from fighting, is reluctant at first, but agrees when they receive a letter from Ramsay, revealing he has killed his father and is holding their brother, Rickon Stark, hostage, and demanding Sansa's return on threat of murdering the wildlings and Rickon before Sansa being gang-raped and Jon himself killed.

Raising an army
With the Karstarks and Umbers having declared for Ramsay due to their anger at Robb and Jon, respectively, Jon and Sansa are forced to recruit the meager forces of the minor Houses to aid them. They are successful in persuading House Mazin and House Mormont to join them, while Robett Glover, head of House Glover, refuses to declare for them, citing his hatred for the wildlings and his anger at Robb for having failed to protect them from Balon Greyjoy's invasion of the North. Though they are still heavily outnumbered, Jon is adamant that they attack Winterfell as soon as possible to prevent Ramsay from gathering more men or the weather turning against them.

The Battle
"Thousands of men don't need to die. Only one of us."

- Jon Snow to Ramsay Bolton.

In the books
In the A Song of Ice and Fire novels, no such battle has taken place yet. The fifth and most recent novel, A Dance with Dragons, ends in the first stages of Stannis Baratheon's attack on Winterfell. Given that season six onwards is based on an outline of what will happen in future novels, the battle (or a slightly different version of it) may take place in either The Winds of Winter or A Dream of Spring.