Explanation:
The Red Wedding occurs in the third book, A Storm of Swords. It takes place at the Twins, a castle belonging to House Frey. The battle takes place during a wedding feast and is orchestrated by Walder Frey. The Starks have come to the Twins under false pretenses—they were invited to Riverrun, but in reality they were lured there to be slaughtered.
The event is significant for two reasons: firstly, it marks an end to one of the most important families in Westeros—the Starks—and secondly, it marks a turning point in the series that has been referred to as “Game of Thrones”‘ darkest moment.