The episode takes place largely during a wedding celebration at The Twins, the castle of House Frey. The wedding is between Edmure Tully and Roslin Frey, intended to seal an alliance between House Stark and House Frey following Robb Stark's breaking of a previous marriage pact.
To directly answer the query: The Red Wedding takes place in It remains a benchmark moment for the series and for prestige television drama. what episode is the red wedding