The Red Wedding is a short sidequest which can be start by seeing a large group of wealthy citizens in front of the church of Eoforwic (York). Although the prequisites are ambiguous, this quest is probably triggered when some progress is made in the Northumbria storyline questline. After the group has finished the wedding ceremony at the church, they towards one of the houses to start the wedding feast. Your greedy hirdman will suggest that you may have an opportunity to rob the attendants when they are drunk enough.
The objective of this quest is to go to the house when it's at least dusk and rob them. From you group, select three hirdman to enter the house with the thegn. When you enter the house, the bride and groom will notice you and ask you to leave, which leads to combat. Since all guests don't wear armour or only have their knive as weapon, they are no match for you. After the battle, you can loot each guest to gain 150, 200 or 250 valuables and earn at least 1,500 valuables. The loss in reputation doesn't change if all the guest were incapacitated due to non-lethal mode though, so you can either kill or incapacitate them all.
The title is a reference to the event in the book series A Song of Ice and Fire and the TV show based on the books, Game of Thrones. Links: https://iceandfire.wikia.com/wiki/Red_Wedding and https://gameofthrones.wikia.com/wiki/Red_Wedding.